Multiple color gradients in the fire/explosion engine?

[olofson]

Support for multiple colors, for more interesting explosions...?

We could implement this as multiple (2-3) layers, that are combined through a 2D or 3D palette LUT. The prior stages would be handled and optimized separately, which means minimal performance impact, as long as we're not actually using multiple layers at once over large areas - which may well happen if we mix different types of enemies without restrictions, so this has game design implications.

Maybe it's sufficient to just extend the temperature/color range, so we can have some explosions look and behave differently that way? Maybe some kind of hybrid with two hardwired layers that are combined in some interesting way?

(Related to #198.)