[Coco] video modes for diagnostic cart

[Patrick Ulland]

> Well, I hate to be “that guy” suggesting things and not being able to do the work, but…
>
> …having a test for “all known” GIME modes would be useful for folks working on emulators. Not sure how that would be approached, but things like the scroll offset register stuff, and things like the “64/128 column” screens and such are a mystery except for a handful of programs that used them. Stuff like that seems to break emulators early on. A way to test could be useful for FPGA, emulation, etc.
>
> 	— A
>
>
> As a hardware guy, I've got to help throw things against the wall. GIME replacements are a growth industry since you can't buy one new. Which also makes them a moving target. Maybe a GIME tester runs after the Diag cart verifies CoCo is healthy enough to load tape/disk? That code could start as an outline of most used functions, expandable to enclude oddball modes, and GIME X/Y/Z could add tests for new functions.
-rick