In aa4e18740a219b I mistakenly thought that I could add the variables
back in and that it would automatically cull variables that aren't used,
but that wasn't the case -- the shader parser always checks to see
whether parameters were set, and if they're not, it'll fail. This fixes
an issue where the shader would try to access parameters that are no
longer needed and fail due to the shader parameter check.
YUV-based shader support has been removed (due to the fact that no
sources ever use YUV shading) so there's no reason to keep around the
YUV processing code.
5edc0c9580