upipe-gl: pass uref to GL_RENDER probe.

RENDER first needed width/height, then aspect ratio.
Let's send the whole uref instead.
upipe-av: avcdv sets aspect ratio (if present)
Merge branch 'master' of git://github.com/cmassiot/upipe
avcdv: be compatible with libav

Don't use frame->{format,width,height} in get_buffer;
use context->{pix_fmt,width,height} instead.