webm/theora/yuv2rgb: Fix YUV conversion issues and add BGRP WEBM handling. (Now clang-format friendly.)

This should fix the various issues with colours in Ogg Theora and WEBM playback.
(A reference project is attached to PR #26051, which this commit should be part of.)

This version of the commit, rather than moving x->RGBA handling into libsimplewebm, uses a colourspace field added to libsimplewebm by a PR there.
Thus, the commit that precedes this should be the synchronization & cleanup commit for that.

Also, this version is now clang-format friendly. I hope.