Eigen only introduced rsqrt() in 3.3. Eventually, we'll switch back to rsqrt(), or even include Eigen directly under 3rdparty/.

This allows to completely create a model in Python from numpy matrices, and store with save_model.

This was in the past used for converting a small BFM json model to a cereal binary. However, it was never satisfactory.
I think it's also used to import the blendshapes from Matlab into C++, so we'll need it again at some point. However at the moment it does not compile as we don't have a json serialize() (cereal) function for Eigen::Matrix.

For some reason, not all changes got staged in the last commit. This is the full set of changes.

Actually, we still read into an OpenCV matrix - should change that in the future to directly read into an Eigen::Matrix.

Also removed division by 1000 of the mean. I don't think it makes sense to make this modification. Leave it as it is in the original.

This is a big step towards being not requiring OpenCV anymore as a dependency.
All models will have to be converted and re-downloaded.

* PcaModel: Removed the RNG from the class, and added a RNG parameter to the draw_sample method
* PcaModel: Added a get_eigenvalues() accessor that returns all eigenvalues
* MorphableModel: Changed CEREAL_CLASS_VERSION from 0 to 1 (and added a version check to the serialize method)
* MorphableModel: Changed texture_coordinates from cv::Vec2f to array<double, 2>
* Added binary Eigen::Matrix serialisation for cereal
* Changed Blendshape to Eigen::Vector as well
* Adjusted blendshape_fitting.hpp accordingly - and it now takes most parameters by const&
* Adjusted fitting.hpp accordingly - now also takes most parameters by const&
* Adjusted linear_shape_fitting.hpp: It's not totally converted, still using cv::Mat for the core algorithm

It hasn't had a method to return a Mesh in quite a while I think.

Hopefully this is not an issue anymore.

CMake changes (built in windows and linux):
* Include newest eigen3 CMake file
* Make CMake scripts work with OpenCV 3.1

The diff looks really weird because I clang-formatted the whole document. But there should be no changes in extract_texture, except for tabs->spaces.

* Can now return any cv::Mat to Matlab
* Can convert 3 and 4-channel images from Matlab to cv::Mat

1) Fixes for OpenCV > 2.4.3
2) Update to newest eigne3 cmake file (support in environment variables).

This enables defining multiple functions per Matlab mex module

Can return 4-chan uchar matrices (isomaps) back to Matlab, and convert 3-chan uchar matrices (images) from Matlab to C++.

These are required for extract_texture in Matlab.
Maybe the better strategy would be to add a constructor to RenderingParameters and not have all these setters.

* Convert Frustum back and forth
* Convert RenderingParameters back to C++
* Convert glm::mat4x4 back to C++

Includes my multidim-array PR plus a fix for the overload resolution in ::to/at