The RTree cannot work with NaN coordinates. So instead of
preparing a vertexFeature before we know a coordinate, we now
lazily create the vertexFeature.
In combination with the additional squaredDistanceToSegment
check we can ensure that a future vertex is only created on one
segment per feature, and only on the segment that it appears on.
This change also renames the misleading dragVertices_ member to
dragSegments_.
This is a first draft. The way geometry changes are handled for
now is a bit clumsy. Both updating the feature cache RTree and
making the layer aware of feature and geometry changes could be
handled in a smarter way if these changes would be made through
the layer instead of directly on the geometry or feature.
