quartz: Actually use the window background PATTERN color

Before we used a window's background color, which resulted in corrupted
display in some cases, presumably because we didn't reset the active
pattern. This patch seems to eliminate the observed corruption.
(cherry picked from commit 0e42cf81f1)

gdk/quartz/GdkQuartzView.c

@@ -92,7 +92,7 @@
        */
       [NSGraphicsContext saveGraphicsState];
 
-      [[[self window] backgroundColor] setFill];
+      [[NSColor windowBackgroundColor] setFill];
       [NSBezierPath fillRect:rect];
 
       [NSGraphicsContext restoreGraphicsState];