Ensure that if method=scale the returned thumbnail is no bytesize-larger than the original image

[Sorunome]

This is to prevent the resulting thumbnail to be larger than the original image, which is unneeded.

[turt2live]

This check would probably be better placed around here:
https://github.com/turt2live/matrix-media-repo/blob/f7546a732c1e00ea24eec73a6ceed5e1e151630a/thumbnailing/thumbnail.go#L43

If the resulting stream (buffered into memory because memory is cheap in this area) is larger than b, return b instead. It should get magically de-duped by the upper layers.

[Sorunome]

Another potential issue soru thought about is how to handle animated vs non-animated: if the client requests a non-animated thumbnail but the source image is animated.

In general the thumbnail should be smaller in bytesize, but....what if not?