Fix T51950: Abnormally long Cycles OpenCL GPU render times with certain panoramic camera settings

The problem here was that when a "invalid" path is generated by the panoramic camera, it was tagged
as RAY_TO_REGENERATE with the intention of generating a new path in kernel_buffer_update.

However, since that state was not handled in kernel_queue_enqueue, kernel_buffer_update did not
process the path which resulted in an infinite loop.

Cycles: Add missing split kernel to CPUDevice

Fix T51909: Cycles: Uninitialized closure normals for the Hair BSDF

As the title says, the normal wasn't set for the Hair BSDF because it wasn't
needed before. However, the denoiser uses it to store the feature passes, so
it needs to be set now.

Fix T51836: Cycles: Fix incorrect PDF approximations of the MultiGGX closures

The PDF of the MultiGGX sampling is approximated by the singlescattering GGX
term as well as a scaled diffuse term that makes up for the energy in the
multiscattering component that's missed by GGX.

However, there were two problems with the glossy terms: The diffuse term missed
a normalization factor, and the singlescattering term was not properly scaled
down based on the albedo estimate.

The glass term was completely wrong and has been rewritten. It uses the fresnel
factor to weight reflection vs. refraction and uses the glossy MultiGGX model
for reflection.
For refraction, the correct singlescattering term is now used, and a new
albedo approximation is used that was derived by evaluating GGX albedo for
roughnesses from 0 to 1 and IORs from 1 to 3 and fitting numerical
approximations to it. The resulting model has a mean relative error of 9e-5,
but could probably be simplified without losing noticable accuracy in the
final render.

The improved PDFs help with glossy highlights (due to better light sampling vs.
closure sampling MIS) and fix the situation described in T51836 where mixing
MultiGGX with other closures (as it happens in e.g. the Principled
BSDF) causes incorrect darkening.

Cycles: Fix excessive sampling weight of glossy Principled BSDF components

If there was any specularity in the Principled BSDF, it would get a sampling
weight of one regardless of its actual impact.

This commit makes Cycles estimate the contribution of the component and adjust
the weighting accordingly, which greatly improves the noise characteristics of
the Principled BSDF in many cases.

Note that this commit might slightly change the brightness of areas when using
MultiGGX and high roughnesses, but the new brightness is more accurate and
closer to the result of Branched Path Tracing. See T51836 for details.

Differential Revision: https://developer.blender.org/D2677

Fix unreported: Copy-pasting nodes crashes when they have an undefined type

Fix unreported: Copy-pasting nodes crashes when they have an undefined type

Cycles Denoising: Add more failsafes for invalid pixels

Now, when there is no usable neighboring pixel for denoising, the noisy value
is preserved instead of producing a NaN.
Also, negative results are clamped to zero.

Note that there are just workarounds that don't fix the underlying problems,
but these issues are very rare and I'm not sure if it's even possible to fix
the underlying problems without introducing a significant slowdown or quality
decrease in other situations.
Because of that and since 2.79 is happening very soon, I just went for these
workarounds for now.

Cycles: Fix denoising passes being written when they're not actually generated

Cycles Denoising: Speedup reconstruction by skipping near-zero weights