Cocoa: Mark the View as opaque

Default is NO.

Cf. http://developer.apple.com/mac/library/documentation/Cocoa/Conceptual/CocoaPerformance/Articles/CustomViews.html

Based on patch by Juha Riihimäki.
Signed-off-by: NAndreas Färber <andreas.faerber@web.de>
Cc: Juha Riihimäki <juha.riihimaki@nokia.com>
Cc: Alexander Graf <alex@csgraf.de>
Cc: Mike Kronenberg <mike.kronenberg@kronenberg.org>
Signed-off-by: Nmalc <av1474@comtv.ru>

cocoa.m

@@ -305,6 +305,11 @@ static int cocoa_keycode_to_qemu(int keycode)
     [super dealloc];
 }
 
+- (BOOL) isOpaque
+{
+    return YES;
+}
+
 - (void) drawRect:(NSRect) rect
 {
     COCOA_DEBUG("QemuCocoaView: drawRect\n");