GLES: Prevent race condition on shutdown.

emuThreadState might become STOPPED before the last frame has been run,
which can cause WaitUntilQueueIdle to hang.

It's simpler just to wait until StopThread() is called, since it now is.
This will line up properly with run_.

Windows/EmuThread.cpp

@@ -244,9 +244,9 @@ void MainThreadFunc() {
 
 	if (useEmuThread) {
 		EmuThreadStop();
-		while (emuThreadState != (int)EmuThreadState::STOPPED) {
+		while (graphicsContext->ThreadFrame()) {
 			// Need to keep eating frames to allow the EmuThread to exit correctly.
-			graphicsContext->ThreadFrame();
+			continue;
 		}
 		EmuThreadJoin();
 	}