[python] Generate GIF animations about loopygen

[Montspy]

I tried layering GIFs with ffmpeg and was successful:

ffmpeg -y -i dancer.gif -ignore_loop 0 -i sunset.gif -filter_complex "[0][1]overlay=format=auto:shortest=1,split[s0][s1];[s0]palettegen[p];[s1][p]paletteuse" output.gif

Overlay filter docs: https://ffmpeg.org/ffmpeg-filters.html#overlay-1

[Montspy]

For metadata generation of MP4s, we should generate a GIF out of it for image until explorer can handle non-images in image, or until we get marketplace

[Montspy]

Turns out MP4 (aka h.264) does not support alpha channels.
The recommended video format with alpha for web is WebM with VP9 codec. https://pixelbakery.co/recipes/list-of-video-image-formats-in-2021-supporting-alpha-channels
ffmpeg supports WebM with VP9. Example command for conversion with alpha channel:

ffmpeg -y -i sunset.gif  -c:v libvpx-vp9 -b:v 0 -crf 30 -row-mt 1 -pix_fmt yuva420p sunset.webm

-c:v libvpx-vp9 forces the libvpx VP9 codec
-b:v 0 -crf 30 forces variable bitrate with constant quality (good for final export) of 30 (0-63, lower is higher quality)
For intermediate steps, I'm thinking of using -lossless instead. Both for speed and to limit the quality loss with high layer counts
-row-mt 1 is using row multithreading. To be confirmed if this speeds conversions up or not
-pix_fmt yuva420p uses YUVA (YUV+Alpha) 4:2:0 pixel format. 4:2:0 is the most used on the web