avisynth: support pix_fmts added to AviSynth+

A number of new pix_fmts have been added to AviSynth+:
16-bit packed RGB and RGBA
10-, 12-, 14, and 16-bit YUV 4:2:0, 4:2:2, and 4:4:4
8-, 10-, 12-, 14-, and 16-bit Planar RGB
8-, 10-, 12-, 14-, and 16-bit Planar YUVA and Planar RGBA*
10-, 12-, 14-, and 16-bit GRAY variants*
32-bit floating point Planar YUV(A), Planar RGB(A), and GRAY*

*some of which are not currently available pix_fmts here and were
 not added to the demuxer due to this

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>