Support Fake Baked Ambient Occlusion texture

Whilst adding support for the 2nd UV sets (#2850) I realised that we could just support the fake baked Ambient Occulsion texture that @nozmajner wanted without them.

This is quite fake as all it does it take the final textured + lit value and then multiply it by the "ambient" texture that is set up using the "tex_ambi" flag in the material.

Texture's must use the same UV's as all of the others like tex_diff, tex_glow, etc but obviously don't have to be the same resolution.

Pinging @Luomu in case you're interested dude :)

Andy