A cutoff distance in spatial interpolators?

When the sample data covers only a subdomain of the frame (say, land), it is sometimes desirable to avoid extending it too much into lakes and oceans. A solution is to add a cutoff distance (in "raster" pixels, like the blur option).

Left: random walk; right: random walk with a cutoff at 9px (and pixelSize: 0.5).

This can be done in user space with custom interpolators, but seems like a simple option to offer in interpolatorNearest (which would deprecate interpolateNearest), and interpolatorRandomWalk. (I haven't looked at interpolatorBarycentric yet.)

Code patch (to be optimized to avoid Math.hypot and skip unless cutoff is specified):

- W[k] = V[index[ix]];
+ W[k] = Math.hypot(X[index[ix]] - x, Y[index[ix]] - y) <= cutoff ? V[index[ix]] : NaN;

Feature suggested by @HarryStevens

(Slightly related to a generic “clip” feature #1109 —but here we only have point samples, no geojson.)