* Previous attempt to make sprite_accessories datums configurable as to whether they should be blended by ICON_ADD vs. ICON_MULTIPLY failed because ICON_ADD's value is actually zero. do_coloration was also checked to see if the sprite should be colorized at all.
* Thus a datum configured to use ICON_ADD (zero) didn't get colored at all!
* Fix separates the concept of "do/don't colorize" from "blend mode to use when colorizing"