Backends: Metal: Align code more closely with other backends. OSX: Add missing return for mouse pos events.

'#if __has_feature(objc_arc)' was removed as callback gets required reference via ImGui_ImplMetal_GetBackendData() so that weakref is not really needed.

backends/imgui_impl_osx.mm

@@ -620,6 +620,7 @@ static bool ImGui_ImplOSX_HandleEvent(NSEvent* event, NSView* view)
         mousePoint = [view convertPoint:mousePoint fromView:nil];
         mousePoint = NSMakePoint(mousePoint.x, view.bounds.size.height - mousePoint.y);
         io.AddMousePosEvent((float)mousePoint.x, (float)mousePoint.y);
+        return io.WantCaptureMouse;
     }
 
     if (event.type == NSEventTypeScrollWheel)