Add fullbright support for models

[SaintWish]

Could use this ,but it'd probably just be better to wait for the render engine. https://github.com/BlueNightHawk/halflife_fullbright

[tschumann]

Interesting - looks like BlueNightHawk/halflife_fullbright@1794262 works by rendering the model twice - once normally, and then once with each texture overlayed with a blank texture that has a bright white light shone on it.

Incidentally, this is possible with some modifications to studiomdl (tschumann/basis@bc311f7). https://www.the303.org/tutorials/gold_mdl.htm#special shows what it looks like.