r_world: Add a new R_DrawTextureChains_Multitexture_VBO function. Draws lightmapped surfaces with fulbrights in one pass, using the VBO. Requires 3 TMUs, GL_COMBINE_EXT, and GL_ADD.
	R_DrawTextureChains_Multitexture: revert to the way it was before VBO support was added.
gl_texmgr: expose GL_SelectTexture. make the implementation less convoluted and support 3 TMUs.
gl_vidsdl: check GL_MAX_TEXTURE_UNITS
r_brush: only create VBOs if 3 TMUs available

git-svn-id: svn://svn.code.sf.net/p/quakespasm/code/trunk/quakespasm@1035 af15c1b1-3010-417e-b628-4374ebc0bcbd