Merge branch 'gradient-lookup'

Merge branch 'gradient-lookup'

This shoves off 6s of 6s seconds from the gradient lookup phase.
It means quantising the range of values, but in RGB land 4096 or so
values should be safe even for multicolor gradients. Otherwise just
adjust QUANT.

We're down to 16.5 seconds from 30.5 now for the standard example:

\# conformal 0.3+
code = """
w=(z-0.5)/(1-0.5*z)
"""
constraint = """
p=(abs(z)<1)
"""
xl = -1.000000
xr = 1.000000
yt = 1.000000
yb = -1.000000
grid = 1.000000
checkboard = 0
gradient = "Full saturation spectrum CCW"
width = 1024
height = 1024