Fix initial blank screen on macOS Mojave (except splash).

(cherry picked from commit 07aee14dd7)

platform/osx/os_osx.mm

@@ -367,6 +367,8 @@ static Vector2 get_mouse_pos(NSPoint locationInWindow, CGFloat backingScaleFacto
 	bool imeMode;
 }
 - (void)cancelComposition;
+- (BOOL)wantsUpdateLayer;
+- (void)updateLayer;
 @end
 
 @implementation GodotContentView
@@ -377,6 +379,14 @@ static Vector2 get_mouse_pos(NSPoint locationInWindow, CGFloat backingScaleFacto
 	}
 }
 
+- (BOOL)wantsUpdateLayer {
+	return YES;
+}
+
+- (void)updateLayer {
+	[OS_OSX::singleton->context update];
+}
+
 - (id)init {
 	self = [super init];
 	trackingArea = nil;