removed screen clearing from the screensize() function
as this is now done by vgascreenwin().

initialize softcursor backup image with DTransparent
color.

produce a nice and distinctive image in vgascreenwin()
like drawterm: text is now black on white and border
size was reduced from 48 pixel to 30. it draws
a purple background and a distinctive window
frame with the title "Plan 9 Console" wich makes
it more distinct from the cga text console.

vgaimageinit just assigns text and background color
from predefined memblack and memwhite instead of
allocating its own colors.