Add support for MP4 videos

It would be great if Coil could support short MP4 videos. MP4 is now commonly used for the same use case as GIF, however it's much more efficient. Similar to GIFs, we would have to buffer the video into memory so we wouldn't be able to support large/long videos.

This would not be a replacement for ExoPlayer, which should be used to handle long videos and any advanced functionality. Any Coil MP4 support would likely only support start/stop and how many iterations to repeat.