This implementation is a bit hacky but match cycles pretty close.
If pixel size is not enabled, it will use the geom shader to
compute distances between vertices. This will have a cost.
Implementation is a bit hacky in gpu_codegen to make the geom shader works
in an optional manner.
Do note that it does not match cycles implementation.
Also we could precompute the hash per strand before rendering but that would
suggest it's not per engine specific.
If we make the random value internal to blender then it won't be a matter
because other renderers will have access to the same value.
This now can shade actual poly strips that mimics cylinders.
This makes hair coverage exact compared to the line method and result in
smoother fading hair.
This does make the sampling a bit more exact but needs more samples to
converge properly.
The problem was that the depth prepass was using the clip plane but not the shading pass.
During the clipping stage, the triangle is converted to a quad clipped to the given clip plane.
But this introduce subtle changes in the depth when this new geometry is rasterized. Since the shading pass was using an EQUAL depth test, the depth values from the shading pass were not always equal to the depth prepass.
Enabling clipping in the shading vertex shader has a too small impact to require a dedicated shader.
