pc/screen.h		1991/0730		Created.
		1991/1225
		1992/1010		Add hwscreenwrite.
		1992/1013		Add screenupdate, islcd.
		1992/1015		Remove screenupdate.
		1992/1021		Remove islcd.
		1992/1104		Add mbbpt, mbbrect, screenupdate, mousescreenupdate.
		1992/1105		Remove mbbpt, mbbrect, screenupdate, mousescreenupdate.
		1992/1107		Add disable.
		1993/0225		Bug fix?: replace changed with pair of counters.
		1993/0226		Remove Mouseinfp, Cursorinfo.
		1994/0211		Remove mouseupdate, hwscreenwrite.
		1994/0414		Add Cursorinfo.
		1994/0520		Add setcursor.
		1994/0624		Change cursoron to return int.
		1997/0327		Add mouse prototypes.
		1997/1101		Change Cursorinfo to be locked Cursor. Add VGAscr, etc. (Changes from Inferno draw? XXX)
		1998/0209		Add arrow.
		1998/0319		Move prototypes around. Add screensize, screenaperture, vgascreenwin.
		1998/0331		Delete blank line.
		1998/0417		Change mousectl to accept list of fields. Remove global mouseswap.
		1998/0507		Add io address to struct VGAscr.
		1999/0119		Add typedef Cursor, just in case.
		1999/0225		Change mouseaccelerate.
		1999/0320		Change *mouseputc to use Queue.
		1999/1005		New draw changes XXX
		1999/1006		Add hwaccel.
		1999/1009		Add drawhasclients.
		2000/0326		Add VGAscr.blank, hwblank, blankscreen.
		2000/0330		Add vgablank.
		2000/0504		Remove blank line.
		2000/0522		Add blanktime.
		2001/0410		Add addvgaseg.
		2001/0426		Add VGAscr.ovlctl.
		2001/0428		Add VGAscr.ovlwrite.
		2001/0501		Change VGAscr.ovlctl.
		2001/0523		Add attachscreen].
		2001/0527		Change mousetrack. Remove VGAscr.ovlctl, VGAscr.ovlwrite, addvgaseg. XXX 9P2000 import loses changes.
		2001/0908		Add addvgaseg. XXX 9P2000 import lost this.
		2001/1005		Add VGAscr.ovlctl, VGAscr.ovlwrite. XXX 9P2000 import lost this.
		2001/1015		Add panning, setscreenimageclipr.
		2001/1120		Change mousectl to use Cmdbuf.
		2002/0404		Add VGAscr.flush, drawflush.
		2002/1201		Add VGAscr.isblank, drawidletime.
		2002/1205		Add VGAMEM.
		2003/0322		Add VGAscr.overlayinit.