devdraw: fix cmd-r to toggle retina vs. non-retina mode on macOS (#361)

and not unexpectedly quitting an application.

Fixes #360

src/cmd/devdraw/mac-screen.m

@@ -510,7 +510,7 @@ void
 rpc_resizeimg(Client *c)
 {
 	DrawView *view = (__bridge DrawView*)c->view;
-	dispatch_sync(dispatch_get_main_queue(), ^(void){
+	dispatch_async(dispatch_get_main_queue(), ^(void){
 		[view resizeimg];
 	});
 }