Cocoa plugin - fix a resource leak

The patch 916dfcb827 while fixing one problem,
introduced another - leaking CGImage.

Change-Id: I08db6ea9fa97ae3489a0bfa1f93e0d18f3671885
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>

src/plugins/platforms/cocoa/qnsview.mm

@@ -589,6 +589,7 @@ static NSString *_q_NSWindowDidChangeOcclusionStateNotification = nil;
     CGContextRestoreGState(cgContext);
     CGImageRelease(cleanImg);
     CGImageRelease(subMask);
+    CGImageRelease(bsCGImage);
 
     [self invalidateWindowShadowIfNeeded];
 }