Apply CMake CMP0077 policy in a different way

In particular, this ensures they are in GPU memory when GPU rendering us being used.

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.)

This isn't supported in 3.14.3, at least, but list INSERT with a 0 index is
equivalent and is available there.

(PR #73 followup)

CMake improvements

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.

...and add a test that would have caught them.

Implementation of Ahmed and Wonka's "Screen-Space Blue-Noise Diffusion of
Monte Carlo Sampling Error via Hierarchical Ordering of Pixels" sampler
(http://abdallagafar.com/publications/zsampler/).

Final images were never being written(!)

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.

Fixes cmake policy CMP0077 build error on windows

(And then the RNG from there on out.)