compat/media: also support decoding with playbin

Allow rendering/decoding to function when using a pure qtmultimedia
scenario as well as a pure playbin scenario.
Make output buffer releasing more robust.

Origin: vendor
Forwarded: no
Fixes bug lp:1234007, out of index crash for handling media_codec_layer output buffers list.

Origin: vendor
Forwarded: no
Make the decoding/rendering loop much more robust. Fixes lp:1234207