This change makes it possible to have a different geometry used
for labeling obstacle detection to the geometry used for generating
label position candidates.
Also fixes parts of multipolygon features were not treated as
obstacles when "label only largest part" of polygon was checked.
Some inefficiencies in pal were also fixed (eg avoiding adding
features/obstacles to pal rtree indexes when they will never
be used).
Sponsored by City of Uster
