a12v: support alpha

[ami_stuff]

attached sample should have alpha

I converted "utvideo_rgba_720_480.avi" from:

​https://trac.ffmpeg.org/raw-attachment/ticket/2616/utalpha.7z

to a12v.

​https://www.datafilehost.com/d/40be2ba5

C:\>ffmpegx -i D:\a12v.avi out.avi
ffmpeg version N-80270-gd4cd8e7 Copyright (c) 2000-2016 the FFmpeg developers
  built with gcc 4.5.0 (GCC) 20100414 (Fedora MinGW 4.5.0-1.fc14)
  configuration: --prefix=/var/www/users/research/ffmpeg/snapshots/build --arch=
x86 --target-os=mingw32 --cross-prefix=i686-pc-mingw32- --cc='ccache i686-pc-min
gw32-gcc' --enable-pthreads --enable-memalign-hack --enable-runtime-cpudetect --
enable-cross-compile --enable-static --disable-shared --extra-libs='-lws2_32 -lw
inmm -lpthread' --extra-cflags='--static -I/var/www/users/research/ffmpeg/snapsh
ots/build/include' --extra-ldflags='-static -L/var/www/users/research/ffmpeg/sna
pshots/build/lib' --enable-bzlib --enable-zlib --enable-gpl --enable-version3 --
enable-nonfree --enable-libx264 --enable-libspeex --enable-libtheora --enable-li
bvorbis --enable-libfaac --enable-libxvid --enable-libopencore-amrnb --enable-li
bopencore-amrwb --enable-libmp3lame --enable-libfreetype --enable-libvpx --disab
le-decoder=libvpx
  libavutil      55. 24.100 / 55. 24.100
  libavcodec     57. 46.100 / 57. 46.100
  libavformat    57. 37.101 / 57. 37.101
  libavdevice    57.  0.101 / 57.  0.101
  libavfilter     6. 46.101 /  6. 46.101
  libswscale      4.  1.100 /  4.  1.100
  libswresample   2.  0.101 /  2.  0.101
  libpostproc    54.  0.100 / 54.  0.100
[012v @ 0x28dd340] transparency is not implemented. Update your FFmpeg version t
o the newest one from Git. If the problem still occurs, it means that your file
has a feature which has not been implemented.
[012v @ 0x28dd340] If you want to help, upload a sample of this file to ftp://up
load.ffmpeg.org/incoming/ and contact the ffmpeg-devel mailing list. (ffmpeg-dev
el@ffmpeg.org)
Input #0, avi, from 'D:\a12v.avi':
  Metadata:
    date            : 2016-05-02T12:14:03+02:00
    encoder         : Adobe Premiere Pro CC 2015 (Windows)
  Duration: 00:00:00.70, start: 0.000000, bitrate: 331545 kb/s
    Stream #0:0: Video: 012v (a12v / 0x76323161), yuv422p16le(10 bpc), 720x480,
348016 kb/s, 29.97 fps, 29.97 tbr, 29.97 tbn, 29.97 tbc
File 'out.avi' already exists. Overwrite ? [y/N] y
[012v @ 0x2c689e0] transparency is not implemented. Update your FFmpeg version t
o the newest one from Git. If the problem still occurs, it means that your file
has a feature which has not been implemented.
[012v @ 0x2c689e0] If you want to help, upload a sample of this file to ftp://up
load.ffmpeg.org/incoming/ and contact the ffmpeg-devel mailing list. (ffmpeg-dev
el@ffmpeg.org)
[avi @ 0x2c68e80] Using AVStream.codec to pass codec parameters to muxers is dep
recated, use AVStream.codecpar instead.
Output #0, avi, to 'out.avi':
  Metadata:
    ICRD            : 2016-05-02T12:14:03+02:00
    ISFT            : Lavf57.37.101
    Stream #0:0: Video: mpeg4 (FMP4 / 0x34504D46), yuv420p, 720x480, q=2-31, 200
 kb/s, 29.97 fps, 29.97 tbn, 29.97 tbc
    Metadata:
      encoder         : Lavc57.46.100 mpeg4
    Side data:
      cpb: bitrate max/min/avg: 0/0/200000 buffer size: 0 vbv_delay: -1
Stream mapping:
  Stream #0:0 -> #0:0 (012v (native) -> mpeg4 (native))
Press [q] to stop, [?] for help
frame=   21 fps=0.0 q=2.0 Lsize=      81kB time=00:00:00.70 bitrate= 945.8kbits/
s speed=14.9x
video:75kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing ov
erhead: 8.147626%