Remove SceneProcessor interface class (only had one implementation)
Rename ParsedScene -> BasicScene

Make SquareMatrix::Determinant() a stand-alone function.
Remove rarely-used Bilerp() function

The previous implementation repeatedly assumed that Allocators were
thread safe; this is not required for std::pmr allocators, but happened to
be the case when the default allocator is used on the CPU (since new/delete
are thread safe.) For the GPU, the CUDATrackedMemoryResource was
also thread-safe, but it also included some per-thread caching that doesn't
really belong in a memory resource.

Therefore, the WavefrontPathIntegrator (and friends) now no longer assume
that allocators are thread-safe, but instead explicitly use per-thread allocators
in parallel code. CUDATrackedMemoryResource is simplified to be just a
memory resource, and now we use a std::pmr::monotonic_buffer_resource
when we actually want chunked allocation.

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).

Assorted cleanups; no functional changes.

* 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>