[Simh] Improved CRT simulation

[Lars Brinkhoff]

Regarding point size, as a test I unceremoniously hacked point() in
type340.c to add eight more points around the specified coordinates.  I
think this results in a considerably more vivid display, and maybe
closer to the screenshots we have.

I tried to reduce the intensity of the added points, but it seems the
dynamic range of intensities offerend by display.c isn't large enough.
Even the dimmest intensity is quite bright.  I didn't see a quick hack
to make more colors available, but if this approach is in the right
direction I could make it work.

I envision a GPU shader implementation where maybe there's a floating
point buffer for each phosphor, modeling the energy of display areas.
These buffers could match the resolution of the simulated display, or
they could be much higher resolution for a slightly better result.  A
separate pass would render these buffers to the screen.