Support animated WebP conversion

[rob93c]

When the bot receives an animated WebP it fails converting it because Scrimage is unable to handle it. Prior to Scrimage, the bot was able to process them via TwelveMonkeys but it didn't generate a video sticker (only the first frame was converted to an image).

Sample file: https://mathiasbynens.be/demo/animated-webp

Note: FFmpeg is unable to decode animated WebP at the moment (the issue opened on their side)