threshold hue value better

[benfoxall]

• the hue value threshold should wrap around, rather than being constrained to the max/min. (easier to set value+margin rather than min/max-thresh)
• would be nice to think in terms of 0->2π rather than mapping to 0->255

[rjmunro]

0 to 2**n-1 is the best way to measure an angles if you use unsigned integers of length n because the overflow does the right thing when you add or multiply them.

Doing this in JavaScript is a bit tricky, though.