If not given, the mean will be used, which is the right thing in that case.

This is used for example when we fit blendshapes and want to fit the PCA shape given current blendshape parameters.

See GitHub issue #4 for details.

This runs 10-30% faster (and is potentially more stable).

Based on the linear landmark fitting.
The regularisation is set conservatively, in the experiments so far even a zero value yielded good results.

It calls the two existing functions, so it requires less code on the application side.

Also added a few notes to the documentation

- Updated ibug2did.txt with a description of the contour indices
- Added a file describing the left and right model contours (the face outline) on the reference model
- Caveat: The implementation is a bit poorly documented for now

* texture is now correctly a boost::optional

Notes:
* Removed the runtime-check in get_mean_at_point()
* I couldn't find out whether ::rowRange has a bounds check in debug builds. Therefore, added an assert.

mainly on the texture interpolation methods

Fixed last commit, added helper functions for glm/Mat and get_4x4_modelview_matrix(fitting::OrthographicRenderingParameters

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()