This is @pierremoreau's suggested fix for issue #100.

I can confirm everything still works on Linux with this fix, but have not
checked the Windows+GPU build.

Issue #104.

Multiple (mostly minor) changes:

- Time is removed from the ray weight computed by the RealisticCamera
  (it's handled by the PixelSensor).
- The "fnumber" and "c" parameters have been removed from the
  PixelSensor's parameters.
- The 683 scale has been removed from both the PixelSensor imaging ratio
  and from SpectrumToPhotometric() (so a no-op, mathematically.)
- Bugfix: Medium implementations keep a separate scalar scale for their
  emission, which is now normalized like regular light sources. This will
  cause image differences.

  diffs for scenes with volumetric emission.

(Only makes sure everything compiles; doesn't run tests.)

* try 2

* ubuntu20.04

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* rgb2spec speedup

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Superceded by support for exporting to pbrt-v4 in assimp.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