Used by the routine at set_draw_objs_overlapped.

Fills in the four bytes that say where an object's sprite lands in the screen buffer: the pixel position at +$1A and +$1B (from calc_pixel_XY) and the size at +$18 (width in bytes) and +$19 (height in pixel rows). The width is the sprite's own, plus one byte when the x position is not a multiple of eight, because a sprite shifted across a byte boundary spills into the next byte.

flip_sprite also turns the sprite the way the object's flip bits ask for. When the object's sprite is the empty one (types 0 and 1 both use it), flip_sprite discards this routine's return address and goes straight back to the caller, so the size bytes keep their previous values. That is what an object that is vanishing wants: the area it last covered is still the area to clear.