Convert DX12 example to use explicit context API

--
This commit is part of the conversion of Dear ImGui toward explicit context API.
This commit is supposed to be applied *after* make_explicit_imgui.py has been run.
--

examples/example_win32_directx12/main.cpp

@@ -83,18 +83,20 @@ int main(int, char**)
 
     // Setup Dear ImGui context
     IMGUI_CHECKVERSION();
-    ImGui::CreateContext();
-    ImGuiIO& io = ImGui::GetIO(); (void)io;
+    ImGuiContext* ctx = ImGui::CreateContext();
+    ::SetWindowLongPtrW(hwnd, GWLP_USERDATA, reinterpret_cast<LONG_PTR>(ctx));
+
+    ImGuiIO& io = ImGui::GetIO(ctx); (void)io;
     io.ConfigFlags |= ImGuiConfigFlags_NavEnableKeyboard;     // Enable Keyboard Controls
     io.ConfigFlags |= ImGuiConfigFlags_NavEnableGamepad;      // Enable Gamepad Controls
 
     // Setup Dear ImGui style
-    ImGui::StyleColorsDark();
+    ImGui::StyleColorsDark(ctx);
     //ImGui::StyleColorsLight();
 
     // Setup Platform/Renderer backends
-    ImGui_ImplWin32_Init(hwnd);
-    ImGui_ImplDX12_Init(g_pd3dDevice, NUM_FRAMES_IN_FLIGHT,
+    ImGui_ImplWin32_Init(ctx, hwnd);
+    ImGui_ImplDX12_Init(ctx, g_pd3dDevice, NUM_FRAMES_IN_FLIGHT,
         DXGI_FORMAT_R8G8B8A8_UNORM, g_pd3dSrvDescHeap,
         g_pd3dSrvDescHeap->GetCPUDescriptorHandleForHeapStart(),
         g_pd3dSrvDescHeap->GetGPUDescriptorHandleForHeapStart());
@@ -138,49 +140,49 @@ int main(int, char**)
             break;
 
         // Start the Dear ImGui frame
-        ImGui_ImplDX12_NewFrame();
-        ImGui_ImplWin32_NewFrame();
-        ImGui::NewFrame();
+        ImGui_ImplDX12_NewFrame(ctx);
+        ImGui_ImplWin32_NewFrame(ctx);
+        ImGui::NewFrame(ctx);
 
         // 1. Show the big demo window (Most of the sample code is in ImGui::ShowDemoWindow()! You can browse its code to learn more about Dear ImGui!).
         if (show_demo_window)
-            ImGui::ShowDemoWindow(&show_demo_window);
+            ImGui::ShowDemoWindow(ctx, &show_demo_window);
 
         // 2. Show a simple window that we create ourselves. We use a Begin/End pair to create a named window.
         {
             static float f = 0.0f;
             static int counter = 0;
 
-            ImGui::Begin("Hello, world!");                          // Create a window called "Hello, world!" and append into it.
+            ImGui::Begin(ctx, "Hello, world!");                          // Create a window called "Hello, world!" and append into it.
 
-            ImGui::Text("This is some useful text.");               // Display some text (you can use a format strings too)
-            ImGui::Checkbox("Demo Window", &show_demo_window);      // Edit bools storing our window open/close state
-            ImGui::Checkbox("Another Window", &show_another_window);
+            ImGui::Text(ctx, "This is some useful text.");               // Display some text (you can use a format strings too)
+            ImGui::Checkbox(ctx, "Demo Window", &show_demo_window);      // Edit bools storing our window open/close state
+            ImGui::Checkbox(ctx, "Another Window", &show_another_window);
 
-            ImGui::SliderFloat("float", &f, 0.0f, 1.0f);            // Edit 1 float using a slider from 0.0f to 1.0f
-            ImGui::ColorEdit3("clear color", (float*)&clear_color); // Edit 3 floats representing a color
+            ImGui::SliderFloat(ctx, "float", &f, 0.0f, 1.0f);            // Edit 1 float using a slider from 0.0f to 1.0f
+            ImGui::ColorEdit3(ctx, "clear color", (float*)&clear_color); // Edit 3 floats representing a color
 
-            if (ImGui::Button("Button"))                            // Buttons return true when clicked (most widgets return true when edited/activated)
+            if (ImGui::Button(ctx, "Button"))                            // Buttons return true when clicked (most widgets return true when edited/activated)
                 counter++;
-            ImGui::SameLine();
-            ImGui::Text("counter = %d", counter);
+            ImGui::SameLine(ctx);
+            ImGui::Text(ctx, "counter = %d", counter);
 
-            ImGui::Text("Application average %.3f ms/frame (%.1f FPS)", 1000.0f / io.Framerate, io.Framerate);
-            ImGui::End();
+            ImGui::Text(ctx, "Application average %.3f ms/frame (%.1f FPS)", 1000.0f / io.Framerate, io.Framerate);
+            ImGui::End(ctx);
         }
 
         // 3. Show another simple window.
         if (show_another_window)
         {
-            ImGui::Begin("Another Window", &show_another_window);   // Pass a pointer to our bool variable (the window will have a closing button that will clear the bool when clicked)
-            ImGui::Text("Hello from another window!");
-            if (ImGui::Button("Close Me"))
+            ImGui::Begin(ctx, "Another Window", &show_another_window);   // Pass a pointer to our bool variable (the window will have a closing button that will clear the bool when clicked)
+            ImGui::Text(ctx, "Hello from another window!");
+            if (ImGui::Button(ctx, "Close Me"))
                 show_another_window = false;
-            ImGui::End();
+            ImGui::End(ctx);
         }
 
         // Rendering
-        ImGui::Render();
+        ImGui::Render(ctx);
 
         FrameContext* frameCtx = WaitForNextFrameResources();
         UINT backBufferIdx = g_pSwapChain->GetCurrentBackBufferIndex();
@@ -201,7 +203,7 @@ int main(int, char**)
         g_pd3dCommandList->ClearRenderTargetView(g_mainRenderTargetDescriptor[backBufferIdx], clear_color_with_alpha, 0, nullptr);
         g_pd3dCommandList->OMSetRenderTargets(1, &g_mainRenderTargetDescriptor[backBufferIdx], FALSE, nullptr);
         g_pd3dCommandList->SetDescriptorHeaps(1, &g_pd3dSrvDescHeap);
-        ImGui_ImplDX12_RenderDrawData(ImGui::GetDrawData(), g_pd3dCommandList);
+        ImGui_ImplDX12_RenderDrawData(ctx, ImGui::GetDrawData(ctx), g_pd3dCommandList);
         barrier.Transition.StateBefore = D3D12_RESOURCE_STATE_RENDER_TARGET;
         barrier.Transition.StateAfter  = D3D12_RESOURCE_STATE_PRESENT;
         g_pd3dCommandList->ResourceBarrier(1, &barrier);
@@ -221,9 +223,9 @@ int main(int, char**)
     WaitForLastSubmittedFrame();
 
     // Cleanup
-    ImGui_ImplDX12_Shutdown();
-    ImGui_ImplWin32_Shutdown();
-    ImGui::DestroyContext();
+    ImGui_ImplDX12_Shutdown(ctx);
+    ImGui_ImplWin32_Shutdown(ctx);
+    ImGui::DestroyContext(ctx);
 
     CleanupDeviceD3D();
     ::DestroyWindow(hwnd);
@@ -434,7 +436,7 @@ FrameContext* WaitForNextFrameResources()
 }
 
 // Forward declare message handler from imgui_impl_win32.cpp
-extern IMGUI_IMPL_API LRESULT ImGui_ImplWin32_WndProcHandler(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam);
+extern IMGUI_IMPL_API LRESULT ImGui_ImplWin32_WndProcHandler(ImGuiContext* ctx, HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam);
 
 // Win32 message handler
 // You can read the io.WantCaptureMouse, io.WantCaptureKeyboard flags to tell if dear imgui wants to use your inputs.
@@ -443,7 +445,14 @@ extern IMGUI_IMPL_API LRESULT ImGui_ImplWin32_WndProcHandler(HWND hWnd, UINT msg
 // Generally you may always pass all inputs to dear imgui, and hide them from your application based on those two flags.
 LRESULT WINAPI WndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam)
 {
-    if (ImGui_ImplWin32_WndProcHandler(hWnd, msg, wParam, lParam))
+    LONG_PTR pLong = ::GetWindowLongPtrA(hWnd, GWLP_USERDATA);
+    ImGuiContext* ctx = reinterpret_cast<ImGuiContext*>(pLong);
+
+    if (ctx == NULL) {
+        return ::DefWindowProcW(hWnd, msg, wParam, lParam);
+    }
+
+    if (ImGui_ImplWin32_WndProcHandler(ctx, hWnd, msg, wParam, lParam))
         return true;
 
     switch (msg)