restore 32bit draw index (zig-gamedev#26)

Author: simeks

libs/imgui/imconfig.h

@@ -117,7 +117,9 @@
 // Your renderer backend will need to support it (most example renderer backends support both 16/32-bit indices).
 // Another way to allow large meshes while keeping 16-bit indices is to handle ImDrawCmd::VtxOffset in your renderer.
 // Read about ImGuiBackendFlags_RendererHasVtxOffset for details.
-//#define ImDrawIdx unsigned int
+#ifdef IMGUI_USE_32BIT_DRAW_INDEX
+#define ImDrawIdx unsigned int
+#endif
 
 //---- Override ImDrawCallback signature (will need to modify renderer backends accordingly)
 //struct ImDrawList;