Luma textures are broken with gl_vbo

[a1batross]

Without VBO:

With VBO:

[a1batross]

This feature is also used in GoldSrc.
kz-endo_slide_svn_muffinslides.bsp.gz

A surface using luma texture is located right at spawn:

@mittorn what do you think could be done here? There is a logic for dt_texturenum, I wonder if it could be reused for luma. I tried to do this actually, binding luma texture to third TMU with GL_ADD combiner in place of detail texture, and detail texture moved to fourth TMU.

[a1batross]

(cont) ... but it breaks lighting on studio models somehow.

[a1batross]

Fixed