Add 512kb VRAM support to the PS/1 2121 on-board VGA chip

[Fenix770]

Hello!
I found in the drivers site https://ps1stuff.wordpress.com/documentation/ibm-ps1-model-2121/#SVGA, that the IBM PS/1 2121 on-board VGA chip support 512kb of RAM. The video chip is similar to the included on the PS/2 Model 25SX machine.

It may be intresting to run Windows 3.1 with 640x480 256 color resolution!

[barotto]

I agree.
As a matter of fact I'm working toward adding support for model 2133 emulation, which requires CD-ROM (working on this right now), SVGA, and 80486.
So SVGA with 512kb+ is definitely planned.