libigl in OrcaSlicer.

[!NOTE] This is not the complete libigl distribution! Only the files needed for compiling libigl into OrcaSlicer are included in the OrcaSlicer source distribution. The full libigl distribution can be found at;

https://github.com/libigl/libigl

This directory contains parts of the libigl v2.6.0 (40e7900) source distribution together with a minimal custom CMakeLists.txt file. In particular only the following files from the full libigl distribution are included;

• README.md
• include/igl/* -> igl/*