Encode audio/video

[porglezomp]

For future-proofing, Twitter's video rules:

Video files must meet all of the following criteria:

• Duration should be between 0.5 seconds and 30 seconds (sync) / 140 seconds (async)
• File size should not exceed 15 mb (sync) / 512 mb (async)
• Dimensions should be between 32x32 and 1280x1024
• Aspect ratio should be between 1:3 and 3:1
• Frame rate should be 40fps or less
• Must not have open GOP
• Must use progressive scan
• Must have 1:1 pixel aspect ratio
• Only YUV 4:2:0 pixel format is supported.
• Audio should be mono or stereo, not 5.1 or greater
• Audio must be AAC with Low Complexity profile. High-Efficiency AAC is not supported.

[porglezomp]

640x360 video would give us 28.8 seconds of audio if we treat each pixel as one audio sample, we can round that to 30 seconds if we cut off the very beginning and end of the background.

[porglezomp]

Alternately we could use 640x360 and have the background scroll, and then we could do longer videos.