vdpau: add a convenience function for getting a decoder profile.

Based on the code by Rémi Denis-Courmont <remi@remlab.net> from VLC.
vdpau: add a constructor for AVVDPAUContext.

We will likely want to add new fields to it in the future, so this is
needed to avoid breaking ABI.
lavc: make error_rate a private option of mpegvideo encoders
lavc: schedule unused FF_QSCALE_TYPE_* for removal on next bump
lavc: move MB_TYPE_* macros from avcodec.h to mpegvideo.h

They are mpegvideo-specific and not supposed to be used by callers.
lavc: deprecate FF_MAX_B_FRAMES

We should not arbitrarily decide the maximum B-frame number for all
encoders supported by Libav, each encoder should be able to set its own
limits.
Move av_fast_{m,re}alloc from lavc to lavu.