Disable capturing the mouse in IMGUI code.

We'll need to go back and find a proper solution (especially if we have 
drag-and-drop issues), but this works as a semi-permanent fix.

contrib/imgui/examples/imgui_impl_sdl.cpp

@@ -271,7 +271,11 @@ static void ImGui_ImplSDL2_UpdateMousePosAndButtons()
     io.MouseDown[2] = g_MousePressed[2] || (mouse_buttons & SDL_BUTTON(SDL_BUTTON_MIDDLE)) != 0;
     g_MousePressed[0] = g_MousePressed[1] = g_MousePressed[2] = false;
 
-#if SDL_HAS_CAPTURE_MOUSE && !defined(__EMSCRIPTEN__)
+// Using this code-path causes several issues in Pioneer - when capturing the
+// mouse, the mouse snaps back to the center of the window, and there are some
+// issues with the WM's decoration extents being calculated into the window's
+// size on the Cinammon DE.
+#if 0 //SDL_HAS_CAPTURE_MOUSE && !defined(__EMSCRIPTEN__)
     SDL_Window* focused_window = SDL_GetKeyboardFocus();
     if (g_Window == focused_window)
     {