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.

gdk/quartz/GdkQuartzView.c

@@ -97,7 +97,7 @@
        */
       [NSGraphicsContext saveGraphicsState];
 
-      [[[self window] backgroundColor] setFill];
+      [[NSColor windowBackgroundColor] setFill];
       [NSBezierPath fillRect:rect];
 
       [NSGraphicsContext restoreGraphicsState];