Ported playvid to new libav API.

At least enough so that it compiles and runs properly.
Finished porting playvid to new libav API, got rid of some GCC warnings.
Addedd avresample to playvid CMakeLists.
Migrated qplayvid to new libav API - with a 2 channel audio limit.
Got rid of qplayvid gcc warnings.
Got rid of the multichannel / stride / buffer size hacks.

We are not creating the intermediary decode->resample buffer ourselves
anyway, and we are not pumping out anything else than stereo sound, so
we might as well simplify some of the packed buffer code.
Fix playvid desync due to losing some samples during resampling.

I was limiting the decoder output to as many samples as were in the
input - thus dropping ~10% of the samples at times. D'oh!
Fix qplayvid desync due to losing some samples during resampling.

See bc8f978a5ea10fc7a5adf10b97c3f824dc332034 logmessage.