[Coco] coco II sideborder

[Nick Marentes]

Good work Simon.

This is the technique used to generate overscan on a CoCo as discovered 
by Sock Master.

Set the screen to a wide screen width (320x200), wait for the video scan 
to approach the end of this then switch to a narrow screen width (256x200).

Timed right, the GIME chip's video counters lose their place and miss 
the end of screen, continueing to generate video.

This also allows us to coax interlace video on a CoCo3... but these 
tricks only work on a 1986 issue GIME chip.

Nick