[players] Skip picture attachments in video players

Picture attachments get reported by ffmpeg as a video stream with the flag AV_DISPOSITION_ATTACHED_PIC.
You generally don't want to play these as video.
omxplayer hangs when trying to play a picture as video (as it never gets enough frames to startup).
The players also get confused with a video file with artwork, and try to play the artwork. E.g.

21:23:53  47.576946 T:2831643728    INFO: ffmpeg[A8C77450]:     Stream #0:0: Video: mjpeg, yuvj420p, 1920x1080, 90k tbr, 90k tbn, 90k tbc
21:23:53  47.578365 T:2831643728    INFO: ffmpeg[A8C77450]:     Stream #0:1: Audio: wmav2 (a[1][0][0] / 0x0161), 48000 Hz, 2 channels, fltp, 192 kb/s
21:23:53  47.579159 T:2831643728    INFO: ffmpeg[A8C77450]:     Stream #0:2: Video: wmv3 (Main) (WMV3 / 0x33564D57), yuv420p, 1920x1080, 12000 kb/s, 29.97 tbr, 1k tbn, 1k tbc

I think this is suitable for Gotham if accepted.

This bug was reported fixed with this patch:
http://forum.xbmc.org/showthread.php?tid=184866&pid=1645333#pid1645333