Make subsequent window creation possible

[ice1000]

Context

I'm the author of the Java binding of dear-imgui, and I want to do GUI tests (say, running some GUI windows on a non-headless machine and automatically stop after a few seconds).
But when I was running the second window, I saw:

java: /home/ice1000/git-repos/jimgui/core/jni/imgui/imgui.cpp:3484：void ImGui::NewFrame():
假设 ‘g.IO.DeltaTime >= 0.0f && "Need a positive DeltaTime (zero is tolerated but will cause some timing issues)"’ 失败。

(假设 == assume, 失败 == failed)

After some debugging I found that when I initialize glfw the second time, glfwGetTime is reset to 0 (or, very close to 0) but g_Time (a static global variable) isn't. And DeltaTime is set to current_time - g_Time which is negative under this circumstance.

io.DeltaTime = g_Time > 0.0 ? (float)(current_time - g_Time) : (float)(1.0f/60.0f)

Modifications

• Added ifndef/define/endif to gl2/gl3 example header files
• Re-initialize g_Time in ImGui_ImplGlfwGLX_Init

[ocornut]

@ice1000 I have merged the change of reseting time (and applied it to the GLFW+Vulkan example). Thanks!

I took the liberty to squash the commits and remove the addition of #ifndef etc. blocks. However small, if you submit two unrelated changes please try to explain the rationale for each changes put them into separate commits so they can be cherry picked separately. Generally we try to use #pragma once for this purpose, though I wonder which problem you were trying to solve (consider that the header file is very small and probably only used in one or two place of a codebase.)

Thank again!

[ice1000]

Get it.

Please continue working on this awesome project!