Synchronize avxffms2 with upstream r753

Removes the postproc dependency from avxffms2, as this causes
builds of FFMS2 since last September (when postproc support was
completely purged from FFMS2) to work with avxffms2 at all.

FFMS2 2.17 still works, as the postproc dependency doesn't seem
to have been necessary for it in the first place.
Remove ErrorHandlingExternal extension.
Since the VfW interface was removed, this no longer serves a purpose.
Merge pull request #97 from qyot27/avxffms2_sync

Synchronize avxffms2 with upstream r753