added alternate VGA Chained emulation object for ET4000 SVGA emulation.
I noticed that ET4000 cards store chain-4 256-color pixels differently
than most SVGA clones of the VGA where it actually shifts the address
down by 2 instead of just masking off A0-A1 from the address and then
using A0-A1 for the plane. This alternate VGA Chained emulation is
activated when you run DOSBox with machine=svga_et4000.

Apparently this change fixes the screen-clearing issues in Optic Nerve.
There are no more "leftover" pieces of the previous demo part behind the
text screens.