macOS: Do not use dispatch queue when canceling idle callback (flutte…

…r#27732)

Fixes flutter/flutter#86932

Relying on dispatch queue to bump call on main thread in this instance
is problematic because platform plugin can hold the dispatch queue.
Using performSelectorOnMainThread: instead works, because it is
processed by the CFRunLoop independent of dispatch queue.

shell/platform/darwin/macos/framework/Source/FlutterSurfaceManager.mm

@@ -92,9 +92,11 @@ - (void)ensureBackBuffer {
                             size:_surfaceSize];
     }
   };
-  dispatch_async(dispatch_get_main_queue(), ^{
-    [NSObject cancelPreviousPerformRequestsWithTarget:self selector:@selector(onIdle) object:nil];
-  });
+  [self performSelectorOnMainThread:@selector(cancelIdle) withObject:nil waitUntilDone:NO];
+}
+
+- (void)cancelIdle {
+  [NSObject cancelPreviousPerformRequestsWithTarget:self selector:@selector(onIdle) object:nil];
 }
 
 - (nonnull FlutterRenderBackingStore*)renderBuffer {