qt5webkit: GStreamer DRAIN query support in the video sink

When the sink receives a DRAIN query, clean up the buffers it handles
and signal the player to do the same.

This patch also introduce a change in the player video frame painting
logic, if the video texture update fails then schedule another
repaint, unless the EOS message was received.
Merge pull request #19 from Metrological/gst-drain

qt5webkit: GStreamer DRAIN query support in the video sink