Nightshade: Routine at D255

D255: Move the drawing position a column left and up, past the column's tiles

Used by the routine at DRAW_OUTLINE. For the far faces of an outline, drawn from right to left (DRAW_OUTLINE). STEP_BACK, the common tail, moves IY past the column's tiles, DRAW_Y by DE and DRAW_X 16 pixels left.
EDGE_LEFT_UP	D255	LD DE,$0008	8 lines up
This entry point is used by the routine at EDGE_LEFT_DOWN.
STEP_BACK	D258	INC IY	Past the column's two tiles
	D25A	INC IY	Past the column's two tiles
	D25C	LD HL,(DRAW_Y)	DRAW_Y moved by DE
	D25F	ADD HL,DE
	D260	LD (DRAW_Y),HL
	D263	LD HL,(DRAW_X)	DRAW_X 16 pixels left
	D266	LD DE,$FFF0
	D269	ADD HL,DE
	D26A	LD (DRAW_X),HL
	D26D	RET