Treat UV gen mode 3 as GE_TEXMAP_TEXTURE_COORDS.

Fixes Riviera, #2214.
Use constants for other uv gen mode usages.

And also change them to treat 3 as coords as well.
Merge pull request #3243 from unknownbrackets/uv-gen

Treat UV gen mode 3 as GE_TEXMAP_TEXTURE_COORDS.