[Simh] Display using GPU shaders

[Lars Brinkhoff]

Hello,

FYI, I'm working on a new CRT simulation.  It's not specifically for
SIMH, but that's one of the intended targets.  Maybe it can become an
update for the display code, or maybe an option.

The main difference in implementation is that it runs mostly on the host
GPU using shaders.  As far as I understand, SDL2 has some kind of
support for using shaders, so I believe there are no technical obstacles
against adding my code to SIMH.

I'm trying to use inspiration from physically based models of a CRT.
But I'm a bit out of my depth, so if anyone has any expertise in this
area I'm all ears.