[pal-developer] Some optimizations and bug corrections

[Marc-Antoine Nussli]

Hi PAL folks,

I've used PAL for one project last year. Thus, I had the opportunity to 
make some little corrections and optimizations which bring big 
improvements for the computation time of polygons labeling.

So, here is a patch to include these modifications as well as some 
explanations of the various changes made.
Maxence will surely integrate all, or a part, of the changes in the 
development version of PAL but you can use the patch in the meantime.

Here are the explanations of the changes:
-Use 'const GEOSGeometry' and 'const PalGeometry' instead of non-const 
ones. This allows to use in more general context where you don't 
necessarly have access to non-const Geometry.

-In function extractXYCoord (Util.cpp), possible bugfix to avoid 
removing the last point of a polygon which should always be equal to the 
first point. At least, this(last point=first point) is something which 
seems to be assumed in other functions doing geometrical computations on 
polygons.

-In fucntion splitGeom (Util.cpp), added a compiler option to skip 
butterflying detection. This is a quite long operation on big polygons 
and it can be avoided if we know that the polygons are valid. More 
generally, it may be interesting to have some way to specify whether pal 
can trust the input geometries and thus avoid useless tests/corrections.

-In LabelPosition::setCostFromPolygon (labelposition.cpp), simple 
changes making very big differences... Coordinates where fetched 
multiple times for each features without any reason...

-In Pal::getLayer (Pal.cpp) removed an exception thrown when the layer 
was inexisting. Seemed not very usefull to me to have an exception here. 
Moreover, with the NULL value, we can use this to check if some layer 
exists or not...

Best regards

ma

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