Add smooth surface terraforming tool

[alice-i-cecile]

Create a smooth slope between local minima and local maxima in the selection.

May need a way to tune the radius to avoid false extrema.

The idea is that this should be very helpful for creating natural-feeling hills, valleys and riverbeds.

[alice-i-cecile]

Better idea! Players select any number of control points. Then, we generate a smooth surface that passes through all of those control points, bounded on the XZ plane by the convex hull of the selection.

[alice-i-cecile]

We should be able to accomplish this with a 3D hermite spline.