add and use Poly2Tri for roof triangulation
complex roof: join unecessary (and interfering) colinear segments from outer polygon
get max roof height from ridge/nodes only when roof:height is not set on part
map VectorXZ to TPoint to avoid unnecessary allocations
add point constraints to triangulation
Merge pull request #55 from hjanetzek/poly2tri

add poly2tri library and use it for roof triangulation