* try 2

* ubuntu20.04

* fix

* fix

* fix

* fix

* fix

* fix

* rgb2spec speedup

* fix

* fix

* fix

* fix

* fix

* fix

* fix

* fix

* fix

* fix

* fix

* fix

* fix

* fix

* fix

(Related, issues #96, #89, #61, #48, and #41.)

Superceded by support for exporting to pbrt-v4 in assimp.

-> assume k=1 then solve for eta that gives the specified spectrum at
normal incidence.

This works around an ugly exit that happens if a worker thread calls
ErrorExit(), so instead we report this and exit before reading in the main
thread if necessary.

Small cleanups in EqualAreaSquareToSphere():
- use std::sin/std::cos rather than approximations
- replace FlipSign() with with std::copysign

Update README.md

Allow multiply-nested MixMaterials in the GPU path.
Switch to Repeat texture mode for bump maps (Amazon bistro expects this).

Issue #98.

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...