ET4000 vs other SVGA chipset emulation: added code to special case
chained VGA rendering for ET4000. When any other VGA chipset, code
renders linear memory like most do in that 4-pixel groups are rendered
from the first of every 16-pixel group (or 4/8 when taking the CRTC
byte/word/dword mode into consideration). ET4000 emulation however
emulates the fact that the CRTC treats byte and dword modes as
equivalent in chained mode and that we can render linearly for it.