Examples: imgui_impl_opengl3 Fix GLAD error condition (#2059, #2002)

`gladLoadGL` returns 0 on fail, not the other way around.

examples/example_sdl_opengl3/main.cpp

@@ -64,7 +64,7 @@ int main(int, char**)
 #elif defined(IMGUI_IMPL_OPENGL_LOADER_GLEW)
     bool err = glewInit() != GLEW_OK;
 #elif defined(IMGUI_IMPL_OPENGL_LOADER_GLAD)
-    bool err = gladLoadGL() != 0;
+    bool err = gladLoadGL() == 0;
 #endif
     if (err)
     {