Used by the routines at BLIT_SPRITE_MIRRORED and BLIT_SPRITE_FLIPPED.

Eight rotations: each one shifts a bit off the top of A into the carry and back into the bottom of C, so C ends up holding A's bits in the opposite order. That is a sprite byte mirrored, and mirroring every byte of a row while reading the row's bytes backwards mirrors the whole sprite.

It is what saves the game from storing anything twice. A creature facing left and the same creature facing right are one set of bytes and a different drawing routine.