Add option to support LAS files with 8bit color depth. Bugfixes.

[mfbonfigli]

This CR addresses most of the open issues:

• A new flag has been added to support LAS files with colors encoded using 8bit color space.
• The way the geometric error is computed for the first tile in the tree has been changed to provide a more correct estimation of the error. This should prevent issues with the tileset disappearing when zooming out.
• A bug that was preventing the system from processing multiple LAS files in a folder should have been fixed.