Libav integration

The QSV support through libmfx is landed on the master Libav and the windows nightly builds already support it out of box, if you use windows all you need is to download the build and install the MediaSDK.

Building

./configure --enable-libmfx --cross-prefix=x86_64-w64-mingw32- --enable-memalign-hack --target-os=mingw32 --arch=x86_64

Running

Currently hevc, h264 and mpeg2 support is implemented

./avconv -codecs | grep qsv

to list them