Added built-in eta and k spectra for CuZn (brass)
Fixed some small bugs in --upgrade
Improve SampledWavelengths::ToString()
GPU: issue warning on unsupported options (--force-diffuse, etc.)

Mostly minor code motion and clang-format formatting.
RandomPermute4() is now gone, since it's no longer used, however...

Switch to a tabule of permutations -> 3x faster (!)
Use 64-bit indices to handle high res / high spp images better.

The CPU integrators no longer write out WIP images during rendering by
default, but this functionality can be enabled using the new
--write-partial-images command-line flag.

Noted in issue #85.

Issue #84.

And similarly for the "randomization" option for the Sobol' samplers.
Also, rename XORScrambler -> BinaryPermuteScrambler

Do a full binary Owen scramble and make it available in the Sobol and
PaddedSobol Samplers. It is now the "owen" randomization strategy and
"fastowen" is the fast hash-based scramble (which is still the default.)

We don't want to put zero sized allocations that return nullptr in the
allocations hash table; doing so causes a crash when we prefetch to GPU
memory.

(Not currently used.)

When rendering on the CPU but with a build that supports the GPU, there's
no need to copy the RGB->Spectrum tables into newly allocated memory.

Issue #75.

And issue an error rather than crashing.

Also, added a CHECK in the AliasTable constructor to actively catch the
case of all-zero weights being provided.

Issue #77.

Defining the macro to be nothing in non-debug builds, results in many
warnings about empty statements.