Effects > Natural > Burn: total overhaul

This effect is now pretty damn neat, if I do say so myself!  It's always
fun to work on effects that are entirely unique to PD, and it seems
especially appropriate for something called "PhotoDemon" to provide a
flame effect... ;)

Three new user-controllable parameters and an entirely new algorithms
make this completely different from the old code, and much more
appropriate for inclusion in the project.