vd_lavc: remove lowres decoding

This was a "broken misfeature" according to Libav developers. It wasn't
implemented for modern codecs (like h264), and has been removed from
Libav a while ago (the AVCodecContext field has been marked as
deprecated and its value is ignored). FFmpeg still supports it, but
isn't much useful due to aforementioned reasons.

Remove the code to enable it.
vd_lavc: remove -lavdopts vstats suboption

This printed per-frame statistics into a file, like bitrate or frame
type. Not very useful and accesses obscure AVCodecContext fields
(danger of deprecation/breakage), so get rid of it.
gl_common: move things used by vo_opengl_old.c only to vo_opengl_old.c

Having this in gl_common is confusing.