Wrapped the extraction of a triangle in a lambda and calling all lambdas with std::async

At the moment, only TextureInterpolation::NearestNeighbour is adjusted to work

- extended TriangleToRasterize
- process_prospective_tri()
- added plane class
- added clip_polygon_to_plane_in_4d()

Moved affine-specific rendering to render_affine_detail.hpp, added raster_triangle() for the generic case

Set version to 0.

To facilitate this, added a get_texture_coordinates() method to MorphableModel

Clarified installation instructions for the header include paths

The OpenGL mathematics library (http://glm.g-truc.net/) - it'll be used for its 3D rendering related functions

Also updated the documentation of Mesh.

Additionally: Using '//' for the forward declarations so they don't get picked up by doxygen

This makes all free functions show up in the generated documentation.