app: [macOS] synchronize rendering with Core Animation for smooth resizes

Magic incantations lifted from

https://thume.ca/2019/06/19/glitchless-metal-window-resizing/

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/metal_ios.go

@@ -21,7 +21,10 @@ Class gio_layerClass(void) {
 static CFTypeRef getMetalLayer(CFTypeRef viewRef) {
 	@autoreleasepool {
 		UIView *view = (__bridge UIView *)viewRef;
-		return CFBridgingRetain(view.layer);
+		CAMetalLayer *l = (CAMetalLayer *)view.layer;
+		l.needsDisplayOnBoundsChange = YES;
+		l.presentsWithTransaction = YES;
+		return CFBridgingRetain(l);
 	}
 }