[MSE][GStreamer] Early reset m_isEndReached (EOS) when about to seek

When a video has reached End Of Stream and a (backwards) seek is done,
the media player won't ever complete the seek because the EOS condition
is still in place and no buffers are ever going to be pushed. A
successful seek completion would reset the EOS condition, but that won't
happen since the seek won't even start.

This change resets m_isEndReached as soon as possible (on
willSeekToTarget()), so the seek can happen.

See: #1568 (comment)

Author: eocanha

Source/WebCore/platform/graphics/gstreamer/mse/MediaPlayerPrivateGStreamerMSE.cpp

@@ -183,6 +183,13 @@ void MediaPlayerPrivateGStreamerMSE::pause()
     player->playbackStateChanged();
 }
 
+void MediaPlayerPrivateGStreamerMSE::willSeekToTarget(const MediaTime& time)
+{
+    MediaPlayerPrivateInterface::willSeekToTarget(time);
+    // Don't consider the stream as EOS anymore after a seek.
+    m_isEndReached = false;
+}
+
 void MediaPlayerPrivateGStreamerMSE::checkPlayingConsistency()
 {
     MediaPlayerPrivateGStreamer::checkPlayingConsistency();

Source/WebCore/platform/graphics/gstreamer/mse/MediaPlayerPrivateGStreamerMSE.h

@@ -54,6 +54,7 @@ class MediaPlayerPrivateGStreamerMSE : public MediaPlayerPrivateGStreamer {
 
     void play() override;
     void pause() override;
+    void willSeekToTarget(const MediaTime& time) override;
     void seekToTarget(const SeekTarget&) override;
     bool doSeek(const SeekTarget&, float rate) override;