This controls how fetched texels are combined with the color that was
produced by a preceding texture unit or with the vertex color if it is
the first texture unit.
Currently only a small subset of possible combine modes is implemented
as required by glquake.
The Context and Software Rasterizer now gets the array of texture units
instead of a single texture object. _Technically_, we now support some
primitive form of multi-texturing, though I'm not entirely sure how well
it will work in its current state.
