ImDrawCallback: Allow to override the signature of ImDrawCallback by #define-ing it. This is meant to facilitate custom rendering back-ends passing local render-specific data to the draw callback.

imgui.h

@@ -1771,7 +1771,10 @@ struct ImColor
 //  A) Change your GPU render state,
 //  B) render a complex 3D scene inside a UI element without an intermediate texture/render target, etc.
 // The expected behavior from your rendering function is 'if (cmd.UserCallback != NULL) { cmd.UserCallback(parent_list, cmd); } else { RenderTriangles() }'
+// If you want to override the signature of ImDrawCallback, you can simply use e.g. '#define ImDrawCallback MyDrawCallback' (in imconfig.h) + update rendering back-end accordingly.
+#ifndef ImDrawCallback
 typedef void (*ImDrawCallback)(const ImDrawList* parent_list, const ImDrawCmd* cmd);
+#endif
 
 // Special Draw callback value to request renderer back-end to reset the graphics/render state.
 // The renderer back-end needs to handle this special value, otherwise it will crash trying to call a function at this address.