Maybe illegitimise incorrect image dimensions

Since matrix-org/matrix-spec-proposals#1975 we require width and height parameters in media repo requests ~~but, e.g., matrix-media-repo 500's on width and height of -1~~. Should we clarify that negative dimensions are not allowed and require servers 400 on those?

Read matrix-org/matrix-spec-proposals#1975 and compare its width and height requirements with the media repository request behavior described here. Check how negative dimensions and HTTP 400 responses are specified, then determine whether the Matrix specification needs clarification and what exact server behavior should be documented.

Written by the indexing model from the issue text.