Set vertex colour attr correctly with material system

[slipher]

Cherry-picked from #1406.

Before:

After:

I think this is still not complete actually, as there are also implicitly vertex lit surfaces that don't have CGEN_VERTEX set. Vertex lighting can be automatically selected when there is no lightmap. The computer console in Vega is an example. It uses an image loaded as shader.

[slipher]

The previous screenshots were with full-range overbright. Although this commit makes the material system match the standard renderer, the result is wrong (too dark).

With clamping enabled, the 'before' picture remains the same (too bright). But clamping + this commit looks just right:

[illwieckz]

LGTM.

The reason why it's too dark is because the overbright code is buggy, something that will be fixed with #1406. So actually getting it too dark is what is expected.

[slipher]

I figured out why implicitly vertex lit surfaces were unaffected. It's because the lightMapping shader forces the color attribute to always be enabled, but the generic shader doesn't. So only stages rendered with generic could suffer from the missing attribute. While implicit vertex lighting only happens for lightmapped stages.

Added a commit commenting out a part of the code that doesn't really work.

[illwieckz]

What is “implicit vertex lighting”?

[slipher]

I mean when the shader does not have rgbgen vertex, but vertex lighting is selected because it's a lighting stage of a BSP surface that has no lightmap.

[illwieckz]

LGTM.