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_macos.go

@@ -14,7 +14,11 @@ package app
 
 CALayer *gio_layerFactory(void) {
 	@autoreleasepool {
-		return [CAMetalLayer layer];
+		CAMetalLayer *l = [CAMetalLayer layer];
+		l.autoresizingMask = kCALayerHeightSizable|kCALayerWidthSizable;
+		l.needsDisplayOnBoundsChange = YES;
+		l.presentsWithTransaction = YES;
+		return l;
 	}
 }