Implement eye model

[shoeffner]

This will allow to accurately project the detected pupil into 3D space to eventually cast a ray onto the screen.

[shoeffner]

9607bca implements a crude variant of this, but the model does not seem to work well.

Next steps:
Use assumed orthogonal projection of face and project pupil onto a model eye ball instead of the flat mapping between the two eye corner points.

[shoeffner]

Won't fix until submission, thus closing this.