The macro-less portions of these terrain graphics rules are now the
TRASH_A and LITTER_BASE macros. I tried to the best of my ability to
understand how they are supposed to work together, so this is the naming
scheme and parameter set I found to be the most optimal:
#define TRASH_A TERRAIN PROB IMAGESTEM
Covers the first and second rules, which are used on a single
terrain pattern.
#define LITTER_BASE TERRAIN IMAGESTEM
Covers the third rule, which affects both trash terrains.