Fixes https://github.com/turt2live/matrix-media-repo/issues/344

See https://github.com/turt2live/matrix-media-repo/security/advisories/GHSA-j889-h476-hh9h

Fixes https://github.com/turt2live/matrix-media-repo/issues/312

We make our own ReadCloser+ReadSeeker implementation because a NopCloser actually destroys the seeker implementation.

This needs to be expanded to cover more of what ffmpeg covers. This does not support animation for a few reasons:
* Most clients which will be using a video thumbnail will be using it in such a way where even a GIF isn't supported.
* Clients rely on the thumbnail endpoint returning images, which can lead to resource inefficiencies if we return GIFs.
* Returning more optimized thumbnails is not feasible for compatibility with most clients.

This is a step towards video processing, maybe.

Fixes https://github.com/turt2live/matrix-media-repo/issues/265

Also fixes an issue with apng thumbnails being cropped in multiplying ways.