devdraw: cocoa metal screen adds a delayed update (#270)

The immediate display of the screen sometimes miss the update from
the CPU side memory.  No obvious synchronization mechanism is available.
In order to make sure the screen updates properly, we set needsDisplay
again after 16ms delay to ensure a second screen update.

src/cmd/devdraw/cocoa-screen-metal.m

@@ -212,12 +212,19 @@ threadmain(int argc, char **argv)
 + (void)callsetNeedsDisplayInRect:(NSValue *)v
 {
 	NSRect r;
+	dispatch_time_t time;
 
 	r = [v rectValue];
 	LOG(@"callsetNeedsDisplayInRect(%g, %g, %g, %g)", r.origin.x, r.origin.y, r.size.width, r.size.height);
 	r = [win convertRectFromBacking:r];
 	LOG(@"setNeedsDisplayInRect(%g, %g, %g, %g)", r.origin.x, r.origin.y, r.size.width, r.size.height);
 	[layer setNeedsDisplayInRect:r];
+
+	time = dispatch_time(DISPATCH_TIME_NOW, 16 * NSEC_PER_MSEC);
+	dispatch_after(time, dispatch_get_main_queue(), ^(void){
+		[layer setNeedsDisplayInRect:r];
+	});
+
 	[myContent enlargeLastInputRect:r];
 }