Adds support for extrusion from more than one edge and from inner
lattice legs. Also fixes extrusion from one point. Implemented in two
steps. At first so called "copy intervals" are calculated according to
selected control points. Then those are used to copy control points to
the new surface patch. See the differential revision for a video.
Differential Revision: https://developer.blender.org/D15524
476de3b463