MDA upgrade to Hercules

[spark2k06]

I have started a new development branch to implement the graphical mode of the hercules. As Hercules is actually an improved MDA, I have chosen to take this as a basis and update it:

https://github.com/spark2k06/graphics-gremlin/tree/hgc_dev

The VGA 70Hz mode is difficult to maintain, as it would have to be determined which CRTC values are appropriate when these are modified when accessing the graphics mode, so in principle it would only work more or less well from the RGBI output.

I don't have an original monochrome monitor compatible with this output, but my monitor accepts the timings from the VGA output, and we can see that the implementation works, at least with the recommended CRTC values corresponding to the 720x348 graphics mode:

https://www.seasip.info/VintagePC/hercplus.html

It would be nice if someone could check how it would look like on an original monochrome monitor.

There are other software or games that use different values that do not display correctly, like prehistorik or simcga... and right now I don't know how to solve those cases:

[spark2k06]

Suggested pull request, along with other major Tandy-related enhancements:

#16