Short: Use CPU (not blitter) to clear mem. V3.20

CPUClr		A small hack, inspired by CPUBlit, that replaces the BitClear
		routine of the graphics library with a highly optimized 68020
		(or higher) routine.  This results in about a 60% speed up on
		a 68020 and should be even more on a 68030/68040.  This is
		version 3.20, an update to version 2.0 on disk number 709,
		includes source.
		Author:  Peter Simons