MFC r263885

o Add new vd_driver method to do bitblt with mask, named vd_maskbitbltchr.
o Move vd_bitbltchr vga's driver method to vd_maskbitbltchr.
o Implement new vd_bitbltchr method for vga driver. (It do single write for 8
	pixels, have to be a bit faster).

Sponsored by:	The FreeBSD Foundation