[ptx] checkerboard behind preview?

[Ed Halley]

I was just about to see if I could tackle the background of the
preview.  In keeping with many other image editing programs, I was
thinking of a gray/gray checkerboard, instead of the current flat black.

I could make a boring 8x8 pixel checkerboard, but thought it would be
nice if I could figure out the appropriate coordinates to make each
check represent a given angle, say, 5 degrees FOV.  This could help the
user judge what numbers to stuff into the yaw/pitch boxes, as well.

Thoughts?

-- 
[ e d @ h a l l e y . c c ]