Bug 1961723 [Linux] Invalidate FFmpeg frames if video stream is sought r=alwu

During seek FFmpeg may allocate new dmabuf frames so we need to remove ffmpeg ID's and allocate new surfaces.

Differential Revision: https://phabricator.services.mozilla.com/D248227

dom/media/platforms/ffmpeg/FFmpegVideoDecoder.cpp

@@ -1756,6 +1756,11 @@ FFmpegVideoDecoder<LIBAV_VER>::ProcessFlush() {
 #if LIBAVCODEC_VERSION_MAJOR < 58
   mPtsContext.Reset();
   mDurationMap.Clear();
+#endif
+#if defined(MOZ_USE_HWDECODE) && defined(MOZ_WIDGET_GTK)
+  if (mVideoFramePool) {
+    mVideoFramePool->FlushFFmpegFrames();
+  }
 #endif
   mPerformanceRecorder.Record(std::numeric_limits<int64_t>::max());
   return FFmpegDataDecoder::ProcessFlush();