bugfix: vga 256-color (normal and chain4 S3) rendering updated to check
if the scan line crosses the VGA 256KB limit, and if so, use an
alternate slow path that ANDs each vidmem address with the mask. This
fixes the closing credits in 1994 demo "Attic" by Digital House, prior
to the fix, the scrolling credits were accompanied by a black line
extending from the middle of the scanline to the end whenever the CRTC
drew up to and crossed the 64KB wraparound of VGA chained 320x200x256
mode.