avviddec: Enforce allocate new AVFrame per input frame

... if ffmpeg would reuse the allocated AVBuffer. Reused AVFrame by
the ffmpeg seems to break our decoding flow since the reused AVFrame
holds the initial opaque data (GstVideoCodecFrame in this case), so
we couldn't trace the our in/out frames.
To enforce get_buffer() call per output frame, hold another reference
to the AVBuffer in order to mark the AVBuffer as not writable.

Fixes: gstreamer/gst-libav#62
21 jobs for handle-avframe-reuse in 59 minutes and 9 seconds (queued for 1 minute and 38 seconds)