[Coco] 64 column "low res" text sceen on coco3

[William Astle]

I don't know if this is known or not. I can't believe I'm the first 
person to try poking values into $FF99 while in the 32 column "low res" 
text mode. Anyway, I found it interesting.

Anyway, with the GIME set up for the 32x16 "coco2" text screen, if you 
set $FF99  to $10, you will end up with a 64x16 "coco2" text screen 
which uses 1024 bytes of memory. This screen still supports all the 
"SG4" graphics blocks.

The "lines per field" setting seems to work, too. Setting it to 200 or 
225 lines seems to work as expected. Setting $FF99 to $70 yields a 64x19 
text screen though the bottom 3 rows of the last line are missing. ($60 
would give 32x19.)

If you're messing with this from basic, you'll need to use a program 
like the following to see the result because basic resets all the video 
registers.

10 WIDTH32
20 POKE&HFF99,&H10
30 GOTO30

Note that this is totally distinct from the high resolution 64 column 
text mode.