Examples: OpenGL3+GLFW/SDL: Made main.cpp compile with IMGUI_IMPL_OPENGL_LOADER_CUSTOM (may be missing init). (#2178)

examples/example_sdl_opengl3/main.cpp

@@ -65,6 +65,8 @@ int main(int, char**)
     bool err = glewInit() != GLEW_OK;
 #elif defined(IMGUI_IMPL_OPENGL_LOADER_GLAD)
     bool err = gladLoadGL() == 0;
+#else
+    bool err = false; // If you use IMGUI_IMPL_OPENGL_LOADER_CUSTOM, your loader is likely to requires some form of initialization.
 #endif
     if (err)
     {