fix region edge behaviour

[Xtarsia]

When background mode is none, get_region_uv() will return the +xz edges from 1 or 2 additional region_texels into empty regions to provide filler data for bridging vertices, and normal calculations.

Set NAN instead of 0.0 when collision reads from edge data if adjacent regions are empty

Average sculpt duplicates source entry for average instead of 0.0 at offsets along edges of regions for more intuitive bahvaior.