Currently only works with the --gpu / --wavefront integrator.
WASD-style controls; could use refinement.
Control-R to record frames.

* 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>

Renamed RandomSampler -> IndependentSampler and thus, now it's
"independent" in scene description files rather than "random" to choose
that one.

Includes various other minor changes that don't affect functionality.

No functionality changes.
Cleaned up some of the GPU *WorkItem definitions.

The PMJ02BNSampler now takes a "seed" parameter.
Improved dimension->pmjInstance indexing (issue #36, via @Andrew-Helmer)

This lets the Windows GPU build run efficiently, without the hack of
synchronizing after each kernel. There, the GPUPathIntegrator is now stored
in CPU memory; in turn, the GPU kernels capture *this by value so that they
can access the values of members.