Large thumbnails undershoot the requested size

This issue has been migrated from [#5302](https://github.com/matrix-org/synapse/issues/5302).

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If you request a larger thumbnail size than the largest precalculated thumbnail, you'll get the largest precalculated thumbnail, which could significantly undershoot the requested size.

This will end up looking horrible and blurry as the client then scales up the thumbnail to the size it asked for.

In Riot/Web (as of https://github.com/matrix-org/matrix-react-sdk/pull/2439) we explicitly ask for the original image if the thumbnail exceeds 800x600, but we might do better to handle this more intelligently. Perhaps the solution is just to pregenerate even bigger thumbnails? Or default to serving the original image?