Transparency information gets lost when converting from gray8a to rgba

[Carl Eugen Hoyos]

(Second part of ticket #769)
When converting from gray8a to rgba, the resulting picture is opaque.

$ ffmpeg -s 32x32 -pix_fmt gray8a -i gray8a.yuv -pix_fmt rgba out.png
ffmpeg version N-44015-g5e363bb Copyright (c) 2000-2012 the FFmpeg developers
  built on Aug 30 2012 10:41:45 with gcc 4.5.3 (GCC)
  configuration: --cc=/usr/local/gcc-4.5.3/bin/gcc
  libavutil      51. 70.100 / 51. 70.100
  libavcodec     54. 54.100 / 54. 54.100
  libavformat    54. 25.104 / 54. 25.104
  libavdevice    54.  2.100 / 54.  2.100
  libavfilter     3. 14.100 /  3. 14.100
  libswscale      2.  1.101 /  2.  1.101
  libswresample   0. 15.100 /  0. 15.100
[rawvideo @ 0x1480400] Estimating duration from bitrate, this may be inaccurate
Input #0, rawvideo, from 'gray8a.yuv':
  Duration: N/A, start: 0.000000, bitrate: N/A
    Stream #0:0: Video: rawvideo (Y2[0][8] / 0x8003259), gray8a, 32x32, 25 tbr, 25 tbn, 25 tbc
Output #0, image2, to 'out.png':
  Metadata:
    encoder         : Lavf54.25.104
    Stream #0:0: Video: png, rgba, 32x32, q=2-31, 200 kb/s, 90k tbn, 25 tbc
Stream mapping:
  Stream #0:0 -> #0:0 (rawvideo -> png)
Press [q] to stop, [?] for help
Truncating packet of size 2048 to 1
frame=    1 fps=0.0 q=0.0 Lsize=       0kB time=00:00:00.04 bitrate=   0.0kbits/s
video:0kB audio:0kB subtitle:0 global headers:0kB muxing overhead -100.000000%