ffmpegdec: post QoS messages when dropping a frame

ffmpegdec: post QoS messages when dropping a frame

There seems to be a little bit of non obvious logic where
the QoS logic can decide to not drop decoding a frame and
pass it to ffmpeg, telling it to drop.
In this case, the QoS logic does not drop the frame itself,
but since the frame will end up being dropped, we still
mark it as dropped and post a QoS message.

https://bugzilla.gnome.org/show_bug.cgi?id=657950