[Coco] real-time mouse cursor

[Roger Taylor]

It's also obvious that to eliminate cursor flicker it has to be drawn 
on each screen refresh which is taking up a lot of CPU time.  It's 
probably a waste of time to pursue this further, but if I can move as 
much code into the IRQ service routine and just use FIRQ to increment 
the raster line counter, there might be a trick in that idea somehow.

If there was a way to trigger an interrupt at a certain raster line, 
this would be solved, but still there's 19 sprite lines each of the 
save, draw, and restore code.  I'm doing these in sequence, btw, 
first saving the entire cursor area, drawing it, then erasing it.

Maybe if I save/draw/restore each line instead of the entire cursor, 
my timing will be more uniform to the raster beam and I can pad out 
each line's cycles to 144, but the total cycles needed to draw the 
cursor is still the killer.

I'm using what I think can be improved which is spaghetti code with 
stack blasting, but if it helps to keep it like it is to help with 
automatic padding to 144 cycles per scanline, then it should be ok like it is.

Padding my sprite code to 144 cycles x 19 lines, plus a little 
overhead all happening 60 times a second... oh boy.