Refactored trash terrain gfx macros into a separate file, removed TRI prefix

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.
Move rules for trash terrains up to where they belong in terrain-graphics.cfg