Add DDF special to autocolor image/dynamic lights based on the average color of a reference image

[dashodanger]

This would help with liquid splashes, fireballs, etc, where the flat or sprite in question has changed in appearance from its vanilla form.

For example, if imp fireballs are recolored to green, the dynamic light would still be red/orange unless the DDF for the fireball attack were updated as well. If the dynamic light could be 'told' to use the fireball sprite's average color as a reference, then in theory the light color should be updated automagically.