This is mainly to accommodate fit-model-ceres, where I've used double for all parameters

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It requires some not-so-beautiful conversions in render_affine and the current texture extraction, but they will be superseded by the new renderer soon anyway.

Not sure what's the best place to put them, let's put them here for now.

He updated it quite a few months ago and sent the file to me - now finally committed.

* Added const and non-const overloads. The non-const is needed because of the RNG in PcaModel.
* Renamed a few variables for consistency

This is needed by the json-to-cereal-binary app to read the JSON generated from the Matlab BFM converter script.

Renamed fit_shape_model to fit_shape, since it's fitting the shape in general (including blendshapes), and not only the PCA model.

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

The previous code is non-standard conformant, it was assigning a temporary to a non-const reference.
VS2015 accepts it most likely because of a non-standard extension.

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.