Replace Electron graphics generation with FPGA transcription.

[TomHarte]

This pull request attempts to transcribe the video generation portion of @hoglet67 's https://github.com/hoglet67/ElectronFpga (as filtered through @moogway82 's https://github.com/moogway82/JamSoftElectronULA which removes from that support for VGA, etc, making it easier to parse to my non-VHDL-aware mind) into C++, in order to benefit from all current knowledge about video generation within the Electron FPGA.

It's a very basic translation at present, with little in the way of software-style optimisation.

[TomHarte]

Immediate successes or near-successes relate to the works of @0xC0DE6502 :

(though note that the left edge of the raster bars implies palette changes at least one column too late)

... but Firetrack is frustratingly now slightly broken — at least usually. Possibly some invalidly-initialised state? Work to do.

When broken:

[0xC0DE6502]

Looking good, Thomas! 👍

[moogway82]

Have you tried pressing '1' on Firetrack? I think that might be working actually and it shows a screen like that when the fields are out of sync. I thought my ULA was broken for a while until I was told this 'press 1' trick - it's in the manual for the game too I believe.

(this is very cool btw, and I'd glad that my paring back of hoglets lovely code was helpful here)

[TomHarte]

Fair enough; hitting "1" does indeed resolve the Firetrack issue, and likely I found it to be intermittent due to uninitialised internal state somewhere. So an issue still exists, probably in either my manual transcription of the VHDL into sequential code or exactly how I've adapted the mechanism for splitting time but if I can't figure it out relatively quickly then I'll merge as is, still definitely being a big improvement, and fix in a subsequent effort to avoid the inbox noise for everyone I've @'ed above.