Integrate CPotree into PotreeDesktop

[m-schuetz]

CPotree: C++ backend with operations that javascript isn't quite made for. Region and Profile extraction tool are currently being updated to the Potree 2.0 format. Would be nice to be able to save all points within a clip region to a las file.

TODO

• Save all points in region to a las file
• Save all points in region to a new Potree 2.0 file, with a pruned octree.
• Allow multi-component regions, e.g. elevation profiles with multiple segments.
• Support "inside vs outside" and "any vs all" clipping operations.