Frame Buffer: POINT's offscreen check is not right

[BigEd]

Running lovebug's program at https://stardot.org.uk/forums/viewtopic.php?f=2&t=27265 doesn't work in PIVDU 2 mode. (It's a Langton's Ant program.)

A fairly minimal test in native ARM which involves moving the graphics origin:

*A.
*PIVDU 2
MODE 4
VDU 29,640;512;
PLOT 69,-50,-50
PRINT POINT(-50,-50)

The POINT returns -1 which indicates off-screen, and that's wrong.

[hoglet67]

We should make sure to fix this for Indigo!

[hoglet67]

Fixed in indigo-alpha2