SMS: use VDP's text mode

[hsoft]

It's been on my TODO list for a while. If it falls in somebody's interests, don't hesitate to claim the task.

Currently, Collapse OS on the SMS uses VDP's graphical mode. That means that glyphs are 8x8. According to TMS9918's datasheet, there's a "text mode" where it's possible to have 6x8 glyphs. That would allow us to have more characters per line and make Collapse OS more usable.

I think it would be a good first task for an ambitious hardware person. Not trivial, but doesn't require deep knowledge of the OS and brings immediate and significant value to the project.

[hsoft]

According to https://segaretro.org/Sega_Mega_Drive/Technical_specifications , Megadrive's VDP removed all existing modes, so using text mode would apparently only work on the actual SMS, not on the MD...

[hsoft]

TODO: explore whether it's possible to use MD's mode 5 (and the whole of the 64K VRAM) from compatibility mode. If yes, then it leaves an interesting door open: use text mode on old SMS and use mode 5 on MDs.