【igl nanovg part-2】Core Implementation

[vinsentli]

This is the core implementation.

And this is a prerequisite pull request for igl nanovg(#213).

[corporateshark]

Please add a new CMake option IGLU_WITH_NANOVG (default: off) to control this new IGLU module.

This new option should be declared inside the root file CMakeLists.txt.

[vinsentli]

Please add a new CMake option IGLU_WITH_NANOVG (default: off) to control this new IGLU module.

This new option should be declared inside the root file CMakeLists.txt.

done