Fix MSVC's C4312 warning about casting user texture type to void* in ImageButton (#2080)

diff --git a/imgui_widgets.cpp b/imgui_widgets.cpp
index 56cbf33..89e6f10 100644
--- a/imgui_widgets.cpp
+++ b/imgui_widgets.cpp
@@ -838,7 +838,7 @@
 
     // Default to using texture ID as ID. User can still push string/integer prefixes.
     // We could hash the size/uv to create a unique ID but that would prevent the user from animating UV.
-    PushID((void*)user_texture_id);
+    PushID((void*)(intptr_t)user_texture_id);
     const ImGuiID id = window->GetID("#image");
     PopID();