[GRASS5] v.overlay

[Radim Blazek]

v.overlay intersects 2 area vectors. Each area in the output map
has 3 categories: link to a new table, and categories of both input maps. 
Attribute table also contains categories from input maps.

Problems could appear if Vect_line_intersection() fails or if any
output area is so small that calculated centroid falls outside,
because of finite-precision numbers (double).

Radim