Initializes a new instance of the NormalPointDesc class

Compute normal by averaging containing triangles. TODO: Get from NormalMap instead?

Whenever change Triangles, must invalidate _layers. (though rarely changes, right?)

Via lines. TODO: What about Triangles?

PERFORMANCE: This is likely bottleneck, but don't want to create on a million triangles, nor keep around after mesh reduction is done.

Remove from UsesMe lists, so not involved in ConnectedPoints.

Quickest comparer, when both points are already in the PointDescSet. NOTE: See PointDescSet.MaybeAdd or .FindOrAdd when don't have point in set (have vector).

IX is to the right by one half-step.

IY is down by one half-step.

Sorted list of layers involving this point.

PERFORMANCE: Computing on-the-fly, to save MEMORY.

Free memory of _layers.

Whenever change Triangles, must invalidate _layers.

These triangles use 'this' point.

This lines use 'this' point.