revid: change case of vidgrind variables for codecs to match av

Vidgrind gets given an enum of codec strings however the case doesn’t match the strings we use in AV so comparison is hard to do. We should just use the same case; lowercase. Lowercase is preferable because they can then easily be used in file extensions and url queries without changing case.

At the moment the case mismatch is actually causing a bug where the codecs chosen are rejected and h264 is chosen as default.