Compute surface position on demand instead of storing it

[hannobraun]

(This issue is part of a larger cleanup effort. See #1589.)

SurfaceVertex is the representation of a vertex in surface coordinates. It stores the surface position of the vertex, and references GlobalVertex (which stores the global position). SurfaceVertex is referenced by HalfEdge and these SurfaceVertex references are shared by the neighboring HalfEdges. Since neighboring HalfEdges can be defined by different curves (and floating point numbers are not infinitely accurate), computing the position of a SurfaceVertex from the HalfEdge can lead to slightly different results for the same SurfaceVertex.

Such inconsistencies must be avoided, which is why surface positions are computed once and then stored in SurfaceVertex. However, there is a simpler way to void the inconsistency: By declaring that each HalfEdge "owns" its start vertex but not its end vertex (which is owned by the next HalfEdge in the Cycle), we can give each HalfEdge responsibility for computing the surface position of its start vertex. Then the stored position in SurfaceVertex can be removed and SurfaceVertex becomes redundant (only a reference to GlobalVertex will be left, which can just be inlined wherever SurfaceVertex is referenced).

This issue has some overlap with #1525, which also would result in SurfaceVertex being removed. The issues are not in conflict, however, and in fact complement each other quite well. With #1525 addressed, this issue would become extremely trivial. Addressing this issue would in turn address most of the rest of #1525. Which order this will end up happening in is going to depend on practicalities, and I'll figure this out as I go.