Fixes https://github.com/turt2live/matrix-media-repo/issues/444

Also fixes a memory issue where we'd decode the entire image before deciding that the image is too small for thumbnailing.

see 1168a7f6

https://github.com/matrix-org/matrix-spec-proposals/pull/4034

Fixes https://github.com/turt2live/matrix-media-repo/issues/407
Closes https://github.com/turt2live/matrix-media-repo/issues/412
Closes https://github.com/turt2live/matrix-media-repo/issues/413

Closes https://github.com/turt2live/matrix-media-repo/issues/411
For https://github.com/turt2live/matrix-media-repo/issues/407

To make `go tool pprof` a bit easier to use

For https://github.com/turt2live/matrix-media-repo/issues/413

For https://github.com/turt2live/matrix-media-repo/issues/413
For https://github.com/turt2live/matrix-media-repo/issues/412