Add an option to display contour lines tracing through points with the
same interpolated weight value in weight paint mode. This can be useful
for working on gentle gradients over a relatively high resolution mesh,
where the difference in color between adjacent vertices is very small.

The contour grid has 3 levels of detail going down to step 0.001,
which automatically fade in or out based on the weight gradient.
Fade out works by capping both screen space and weight space line
width, and reducing alpha when the weight space limit is dominant.