quartz: Fix garbage content when windows are initially mapped

The garbage would be visible if any widget enabled the toplevel
NSView's CALayer in order to do custom native rendering.
(cherry picked from commit 92ea94af5f)

gdk/quartz/GdkQuartzView.c

@@ -81,6 +81,25 @@
   if (NSEqualRects (rect, NSZeroRect))
     return;
 
+  if (!GDK_WINDOW_IS_MAPPED (gdk_window))
+    {
+      /* If the window is not yet mapped, clip_region_with_children
+       * will be empty causing the usual code below to draw nothing.
+       * To not see garbage on the screen, we draw an aesthetic color
+       * here. The garbage would be visible if any widget enabled
+       * the NSView's CALayer in order to add sublayers for custom
+       * native rendering.
+       */
+      [NSGraphicsContext saveGraphicsState];
+
+      [[[self window] backgroundColor] setFill];
+      [NSBezierPath fillRect:rect];
+
+      [NSGraphicsContext restoreGraphicsState];
+
+      return;
+    }
+
   /* Clear our own bookkeeping of regions that need display */
   if (impl->needs_display_region)
     {