[Bf-cycles] Hair render settings

[Brecht Van Lommel]

Hi Stuart,

I'd like to enable the cycles hair render without the silly
experimental choice after the 2.68 release, but think the settings
could be simplified before that. I suggest to remove the Custom type,
and instead make the primitive a choice like this with further options
always visible:

* Primitive: Curve, Line, Triangle (low memory usage vs. speed)
* Shape: Ribbon or Cylinder (flat normal towards viewer or rounded)

I think that covers all the different types? Except for the difference
between Triangle Ribbons and Planes, which I don't really understand,
Planes seems to always look better to me so maybe just keep only that?
I don't know if this could be simplified further, if there are any
hair primitives that don't seem particularly useful they could be
removed but all seem to have some advantages.

Further some other settings that could perhaps be simplified:

* Interpolation: can this use the settings in the Render panel or
replace it? There is already a B-Spline and steps, this seems
unnecessary.

* Line Segments
** Method: can this always be accurate?
** Exclude encasing: I'm not sure when this option helps, only found
missing shadows when using this.
** Use tangent normal as default / geometry: I think this could be
left to materials?

* Triangles Planes Smooth option seems to do nothing. Think this could
always be enabled and removed anyway?
* Include Parents: the render panel already has a Parents option that
could be used?


If any users have some suggestions about which settings are useful, or
should be removed, or have their default changed, etc, that feedback
is welcome too.

Thanks,
Brecht