Allow changing gel colors mid-map

[vrad-exe]

Changing the paint_color cvars should take effect immediately, instead of requiring you to restart the map or load a save. This would be important for #127, since maps will want to set the color of the custom gel.

[StefanH-AT]

This is probably related to paintmaps storing the actual paint rgb color on each luxel rather than the paint enum index. We want to optimize this anyways to reduce memory and enable paint on every map