Support Video Files That Contain Embedded Artwork

[complexlogic]

For some file formats (notably MP4), FFmpeg uses an AVStream structure to logically represent embedded cover art. USDX's current video stream selection logic always selects the first video stream reported by FFmpeg. If the first video stream is cover art, USDX will fail to play the video. See #902.

FFmpeg sets a flag AV_DISPOSITION_ATTACHED_PIC to indicate that an AVStream object represents cover art. This PR updates the stream selection logic to reject video streams that have the flag set. This ensures that an actual video stream will be selected for files that contain embedded cover art.

Fixes #902

[barbeque-squared]

Test code courtesy of @complexlogic

import mutagen.mp4
import sys

byte_array = None
with open(sys.argv[2], "rb") as f:
    byte_array = f.read()

file = mutagen.mp4.MP4(sys.argv[1])
file['covr'] = [mutagen.mp4.MP4Cover(byte_array, imageformat=mutagen.mp4.MP4Cover.FORMAT_JPEG)]
file.save()

first param = mp4, second = some jpeg

The code looks fine and I verified that it fixes the problem