Add a UserData pointer so that the callback functions can easily access user state.

Just in case a project has adverse reactions to adding globals or statics in their own code.

imgui.cpp

@@ -332,6 +332,7 @@ ImGuiIO::ImGuiIO()
 
     // User functions
     RenderDrawListsFn = NULL;
+    UserData = NULL;
     MemAllocFn = malloc;
     MemReallocFn = realloc;
     MemFreeFn = free;

imgui.h

@@ -430,6 +430,7 @@ struct ImGuiIO
     // REQUIRED: rendering function. 
     // See example code if you are unsure of how to implement this.
     void        (*RenderDrawListsFn)(ImDrawList** const draw_lists, int count);      
+    void*       UserData;
 
     // Optional: access OS clipboard (default to use native Win32 clipboard on Windows, otherwise use a ImGui private clipboard)
     // Override to access OS clipboard on other architectures.