[Simh] Improved CRT simulation

[Lars Brinkhoff]

Hello,

I'm entertaining the idea of improving the CRT display simulation in
SIMH.  As an example of what I'd like to see, consider these pictures.
The first is from the Type 340 simulation, and the second is a frame
from a film shot at the MIT AI Lab.

https://github.com/larsbrinkhoff/simh/issues/5#issuecomment-446859516

If you read the fine print in the 340 documentation, you'll find that an
a point is .03"-.015" depending on the intensity, compared with the grid
spacing .0091".

Does the SIMH video framwork support using shaders written in e.g. GLSL?