Cycles Hair: Two basic bair shaders added

A new hair bsdf node, with two closure options, is added. These closures allow the generation of the reflective and transmission components of hair. The node allows control of the highlight colour, roughness and angular shift.

Llimitations include:
-No glint or fresnel adjustments.
-The 'offset' is un-used when triangle primitives are used.

Fix compile of last commit

Updates for the Cycle Hair UI. With the following changes
- Removed the cycles subdivision and interpolation of hairkeys.
- Removed the parent settings.
- Removed all of the advanced settings and presets.
- This simplifies the UI to a few settings for the primitive type and a shape mode.

Cycles Hair: Corrected own stochastic hair rng mistake and removed an unnecessary padding variable

- Corrected the Sobol dimension and removed a pad variable in Curve Struct.

Cycles Hair: Strand Minimum Pixel Size

Code is added to restrict the pixel size of strands in cycles. It works best with ribbon primitives and a preset for these is included. It uses distance dependent expansion of the strands and then stochastic strand removal to give a fading. To prevent a slowdown for triangle mesh objects in the BVH an extra visibility flag has been added. It is also only applied for camera rays.

The strand width settings are also changed, so that the particle size is not included in the width calculation. Instead there is a separate particle system parameter for width scaling.

Cycles Hair: Fixed hair shape and inclusion of multiple attributes for triangle primitives

When using triangle primitives this fix enables 'closed tip'.

UVs and vertex colours are added when using triangle primitives for hair.

Two new preset modes have also been included to allow easy access to curves and triangle planes.

Cycles Hair: Addition of render settings and static BVH strand width scaling

Addition of a RNA function to toggle between the hair settings and rebuild the cache. This enables the usage of the render step, child number and full display percentage with f12 rendering.

A scaling to the strand radius has also been added for the static bvh. This only matches up with dynamic for uniform scaling.

A very small fix is included for multiple uvs/vertex colours when using child particles.

Cycles Hair: Multiple vertex colours and UV coordinates

Added export of multiple UV coordinates and vertex colour attributes.

A debugging option to export the strands without using the cache has also been removed.

Cycles Hair: Fix of cardinal curve intersection routine

Fixed some of my own mistakes in the cardinal curve intersection routine.

Cycles Hair: Introduction of Cardinal Spline Curve Segments and minor fixes.

The curve segment primitive has been added. This includes an intersection function and changes to the BVH.

A few small errors in the line segment intersection routine are also fixed.