avisynth: Support video input from AviSynth 2.5 properly.

Uses the 2.5 compatibility header included with the variant of
FFMS2 that uses AviSynth's C-interface. A copy of this header is
now provided in compat/avisynth.

avs_get_row_size_p and avs_get_height_p changed between versions
2.5 and 2.6. Since the avisynth_c.h header that avformat uses
assumes AviSynth 2.6, it would cause 2.5 to crash if given any
kind of real video (the Version() function was known to work,
though).

AvxSynth was unaffected by this issue because, despite being based
on AviSynth 2.5.8 and using 2.5.8's interface version number of 3,
it actually uses 2.6's versions of these functions.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Revert "doc/RELEASE_NOTES: add a note about AVISynth"

This reverts commit 3aa2257d240a5a0eb94014b9113dd91730786886.