[GEM Development] Another driver update

[John Elliott]

  It seems an opportune time to note my latest update to the FreeGEM driver
source:

* Merged changes to build drivers for GSX-86 rather than GEM.

* Fixed a bug in the calculation of Y-coordinate ranges for a polygon,
 which could slow a fill down under rare circumstances.

* Fixed the 3270PC low-res driver so it doesn't get a divide error when 
 setting line width greater than 1.

* Created a 720x540 driver for those chipsets which support it (currently 
 only tested on a Chips&Technologies 82C451).

* There's an 800x600x16 driver which supports VESA and some non-VESA
 chipsets, including Trident, Paradise and C&T 82C451.

* There's a 256-colour driver for the Paradise VGA. It does 640x480x256 if
 the video card has enough memory, 640x400x256 otherwise.

  The reason I can do the Paradise 640x480x256 driver when I can't do a VESA
640x480x256 driver is that the Paradise hardware is more flexible; it lets
me position the memory window at arbitrary 4k offsets, so I can use a 60k
window (corresponding to 96 whole lines) rather than a 64k window 
(corresponding to 102.4 lines).
  I've only tested the Paradise 640x480x256 driver on an XT, on which it was
sloooow.

-- 
John Elliott