PolyPen is very slow if many components are selected

[jlampel]

PolyPen really struggles if a lot of components are selected, like in the example below.

This is probably from iterating over all possible edges on every mouse update to find the best connection. While this isn't something we would encourage users to do and not something we need to explicitly support, there are two things I think we could do to help speed this up so that RF doesn't come to a crawl if a user does this accidentally on a large mesh.

1. We could eliminate any interior verts from the calculation and only look at the boundary loop, since we can assume we don't want to draw a face on top of another face.

2. When the mouse is over a face or right next to an edge, we could assume that the user wants to make a cut instead and not evaluate the connection points at all.

Related to: #1368