Also, add a check that issues an error if there is a MixMaterial that uses
a complex texture to select between the materials (which isn't supported in
the current implementation).

Implement them for GPU textures.
Include the filename for all textures.

Previously, they effectively required a multiple of 4...

Rather than tracking them along with the ray (and incurring the
corresponding bandwidth and memory costs), we always use the
Camera::Approximate_dp_dxy() method, which may mis-estimate them in the
presence of specular reflection and refraction, though generally works
well.

Mostly addresses issue #10.

(And then explicitly instantiate the templates.) This gives roughly a 10%
performance improvement for the kernels that call this on the GPU.

No effect currently other than using more memory (since the provided filter
widths are all zero) and a 10-15% performance loss in the Coated*Material
kernels due to that much more register pressure...

This way, we can not increment the ray depth when it passes through a
surface that markes the boundary between different media, matching the
behavior of the regular CPU path.

In particular, this fixes some undesirable noise in the wavefront
integrator (and different behavior from the regular CPU path) that was due
to the ray depth being incorrectly tracked in scenes with participating
media and then Russian roulette inadvertently being applied after the very
first surface intersection.

- Renamed method and updated signature to not be
  SurfaceInteraction-dependent.
- Inlined implementation and added check for CameraBase implementation to
  improve dispatch performance.

In turn, this gives us valid dzdx and dzdy values with GBuffer film.

Now a MultiWorkQueue templated on the phase function type is used for
enqueuing work items for sampling direct/indirect lighting in media.  With
Henyey-Greenstein as the only phase function currently available, this
reduces to the previous behavior, but it makes it easier to add new phase
functions in the future.

Also piped through time in a few places it was missing in the
medium-related work queues.

Removed another unused BSDF member. Other very minor changes;
functionality unaffected.

Issue #129.

Before we were supplying any hit shaders and asking OptiX for all
intersections, once, along each probe ray. For reasons unclear, results
were darker on the GPU than on the CPU.

This implementation successively finds the closest hit along the segment,
spawning a new ray after each intersection. With this, both CPU and GPU
subsurface scattering images match (modulo noise).

Improvement to 852829c7; rarely,
SampleWavelengths() would compute a lambda that was just slightly above
Lambda_max, which led to NaNs...

Given the same acceleration structure, then the images they generate should
now match exactly (if there is no SSS or participating media, at least.)

via Nathan Vegdahl:
https://psychopath.io/post/2021_01_30_building_a_better_lk_hash and then
subsequent improvements communicated via email.

Assorted cleanups; no functional changes.

Fixes #128.

Relates to issue #100, #96, #89, and #48...

This fixes many of the Windows+GPU crashes.

Issues #96, #100, and more...

* Generalize GPU rendering path to run on both CPU and GPU

Now we have a WavefrontIntegrator that can both run on the CPU (backed by
ParallelFor() for parallelization and pbrt's aggregates for ray
intersection acceleration) and on the GPU (backed by GPUParallelFor() for
parallelization and OptiX for ray intersection on NVIDIA GPUs.)

Beyond generalizing the code, this refactor allows CPU-side debugging and
testing of the wavefront integrator.  Doing so allows further isolation of
the GPU-specific code into a few source files, now just ~2.5k lines of code.

This includes a bug fix in the wavefront medium code to resolve MixMaterials
to one of their constituent materials before enqueuing material evaluation
and shading work.

Note that on the CPU, the wavefront integrator runs 5-10x more slowly than
pbrt's regular CPU integrators, so it is not recommended for regular use...
Co-authored-by: NWenzel Jakob <wenzel.jakob@epfl.ch>

Issue #124.

Fixes #25