Fixes for lighting in custom maps

[res2k]

Background:
After a bit of a while I looked at some custom maps again, specifically some which use very few "baseq2" textures and rely on SURF_LIGHT faces for lighting quite a bit.

The issue:
Suddenly all the walls very way too bright...

Turns out the map author used textures, including the wall textures, mostly for "normal" surfaces, but sometimes for emissive surfaces!
However, the code currently handling light surfaces with materials not specified in the material system sets MATERIAL_FLAG_LIGHT if a material is encountered on an emissive surface (SURF_LIGHT) - so one occurrence of that and suddenly all uses emit light!

Attempt 1:
My first thought was to treat surfaces lacking the SURF_LIGHT as always non-emissive.
However, this breaks quite a few updated materials that add emissive layers on textures that where not used for light emission in the original game.

Attempt 2 (this PR):
This only considers the presence of SURF_LIGHT if the emissive texture was not explicitly given (ie synthesized emissive texture): no SURF_LIGHT → no emission.
Otherwise, no SURF_LIGHT means "fully emissive" (previous behavior).