- made Vulkan Cocoa view opaque

It is recommended to make content view opaque like NSOpenGLView
I didn't notice any difference in performance though

src/posix/cocoa/i_video.mm

@@ -251,6 +251,11 @@ namespace
 	return [self.class.layerClass layer];
 }
 
+-(BOOL) isOpaque
+{
+	return YES;
+}
+
 @end