drawgfx.h: Long-overdue documentation about how to use priority-masked
drawing (aka pdrawgfx); add some useful constants. [Alex Jackson]

(nw)
prehisle.c: Fix issues introduced by pdrawgfx conversion; use
required_region_ptr instead of runtime tag lookup; give gfx regions
more meaningful names; remove superfluous "prehisle"'s and "16"'s
from member names (it's been many years since you could accidentally
plug a handler of the wrong width into an address map and not have
the core catch you at compile time)