Add a libav based generic media extractor.
Fix uninitialized variable in libav extractor.

And warning about passing NULL to an argument that takes an int in
vorbis extractor.
Ensure video size is extracted by libav extractor.

It may be necessary to parse the MPEG bitstream to get width and height
from some videos so if the resolution isn't immediately available decode
enough data to determine it.
Improve detection of media stream properties.

Use the much more comprehensive avformat_stream_info to probe
data from packetized data rather than directly trying to partially
decode frames.
Fixes after review for the libav support