Changed all affected functions and classes accordingly: Mainly the renderer, affine-renderer, and affine texture extraction.

This results in a ~35% speed-up of rendering meshes.

It requires some not-so-beautiful conversions in render_affine and the current texture extraction, but they will be superseded by the new renderer soon anyway.

Fits camera, shape, blendshapes and colour coefficients with a cost function containing a landmark constraint, priors, and minimising the error between the input image's RGB values and the model estimate (rendered at each vertex).

Note: The image cost function cannot be used with the model on GitHub, as it doesn't include the colour PCA.

* Fits the pose (camera) to landmarks using the newly added Ceres cost functions
* Added find_package(Ceres) to CMakeLists.txt

Added a BUILD_UTILS CMake option, defaulting to OFF.

I don't think we need two representations for it, one struct with a type is easier. At the moment, we only use an orthographic camera anyway.

- As before, it should contain all the parameters necessary to render an image using OpenGL, and conform to the OpenGL conventions
- Updated all calling sites
- Added serialisation
- Added a convenience method to save the parameters to a json file

Probably needed because OpenCV needs it, and we depend on OpenCV.

fit-model: Replaced affine camera estimation with nonlinear algorithm (allows direct recovery of the pose angles)

This should make the app easier to understand, i.e. remove anything that's not necessary.

imread has moved from highgui to the imgcodecs module in version 3.0.

imread has moved from highgui to the imgcodecs module in version 3.0.

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

Also updated the documentation of Mesh.

- We now use the same values and formulas as in Smith & Aldrian for the detector 2D variance.
- This changes the ranges of values a bit, so set an equivalent reasonable regularisation default value

Changed the detector variance in the shape fitting from clip-coords to image-coords. This makes the regularisation behave normal again.

Also chose a slightly higher regularisation value to stay a bit closer to the mean face by default.

In the default setting, it links to OpenCV's debug libs, and that doesn't work with the STL of the RelWithDebInfo build.

Also removed a few trailing whitespaces

For path operator +=

Removed all non-header-only CMake stuff and added a custom target for the library headers so they show up in IDEs