This reverts commit b1358dc2.

(This seems to cause some color shifts; reverting pending chasing down.)

via https://psychopath.io/post/2021_01_30_building_a_better_lk_hash

Removed support for double-precision Sobol' samples.

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

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.)

Folded HaltonPixelIndexer back into HaltonSampler, among other small changes.

This is mostly various parts of Physlight moving around a bit in the code, but without changes to functionality.

Small bugfixes in Spectra::D() and the handling of Sensor in GBufferFilm.