imgui_impl_opengl2: Add ImGui_ImplOpemGL2_NewFrame_NoAssert()

Here I'm introducing a new method ImGui_ImplOpemGL2_NewFrame_NoAssert()
for Dear ImGui's OpenGL 2 implementation/

This is apecial edition of ImGui_ImplOpenGL2_NewFrame() for VST plugin editors.
It won't assert if OpenGL backend is not intialized, just return bool state.

This function aims at UI drawing method called on effEditIdle, without
using thread or mutex. As effEditIdle will always be triggered even when
you are closing the editor. At this time, the original
ImGui_ImplOpenGL2_NewFrame() won't continue, then crash with an
assertion.

To use in production, You must check if this method success or not before other steps,
or your editor may crash!

backends/imgui_impl_opengl2.cpp

@@ -104,6 +104,25 @@ void    ImGui_ImplOpenGL2_NewFrame()
         ImGui_ImplOpenGL2_CreateDeviceObjects();
 }
 
+// Special edition of ImGui_ImplOpenGL2_NewFrame() for VST plugin editors.
+// Won't assert if OpenGL backend is not intialized, just return bool state.
+// This can make sure that you can safely call UI drawing method on effEditIdle,
+// without using thread or mutex.
+//
+// To use in production, You must check if this method success or not before other steps,
+// or your editor may crash!
+bool    ImGui_ImplOpenGL2_NewFrame_NoAssert()
+{
+    ImGui_ImplOpenGL2_Data* bd = ImGui_ImplOpenGL2_GetBackendData();
+    if (!bd)
+        return false;
+
+    if (!bd->FontTexture)
+        return ImGui_ImplOpenGL2_CreateDeviceObjects();
+
+    return true;
+}
+
 static void ImGui_ImplOpenGL2_SetupRenderState(ImDrawData* draw_data, int fb_width, int fb_height)
 {
     // Setup render state: alpha-blending enabled, no face culling, no depth testing, scissor enabled, vertex/texcoord/color pointers, polygon fill.

backends/imgui_impl_opengl2.h

@@ -30,3 +30,6 @@ IMGUI_IMPL_API bool     ImGui_ImplOpenGL2_CreateFontsTexture();
 IMGUI_IMPL_API void     ImGui_ImplOpenGL2_DestroyFontsTexture();
 IMGUI_IMPL_API bool     ImGui_ImplOpenGL2_CreateDeviceObjects();
 IMGUI_IMPL_API void     ImGui_ImplOpenGL2_DestroyDeviceObjects();
+
+// Special methods used by VST plugin editor
+IMGUI_IMPL_API bool     ImGui_ImplOpenGL2_NewFrame_NoAssert();