Read Optix path from environment.

This way it doesn't has to be set in CMakeCache.txt every time the
repository is checked out.

Add mouse handling.

Noted by @pbrt4bounty in #246.

On Windows, both CPU and GPU can't write to managed memory at the same time.
Previously, it was possible that the asynchronous creation of the sampler, camera,
film, etc., could still be running when OptiX BVH construction started. This was
especially likely with the Halton sampler + random digit permutations, which take
a while to initialize.

Fixes #246.

Cuda toolkit

Add more recent versions, up to 11.7.0, and remove older ones.
11.2.2 is now the oldest version tested.

Allows removing the workaround for NVIDIA's changed signing keys.

taggedptr_test.cpp adds a test type to check if the fix works as expected.

Fix GPU github actions build due to breakage from NVIDIA changing apt signing keys.

When pbrt is compiled with Float=double, the uint64 used to store the
reversed digits could overflow, which led to all sorts of interesting image
artifacts due to non-uniform samples being returned by the HaltonSampler.

* Reverse direction for PDF_Li to intersect with light

* Fix L computation of directly visible light for LightPathIntegrator

Fixes Issue #239.

Currently only works with the --gpu / --wavefront integrator.
WASD-style controls; could use refinement.
Control-R to record frames.

https://qoiformat.org

(Allows cleaning up some unnecessary complexity in checking that
round trips come out the same.)

Also, slightly simplify Image::WritePNG()

(Thus, for example, imgtool convert foo.png --outfile bar.png now always gives
a pixel-identical result; before the dithering in the reverse conversion would
very occasionally give a different pixel value.)

On failure, it doesn't return NULL, but returns INVALID_HANDLE_VALUE...

This was causing pbrt to cryptically report "The parameter is incorrect"
when it should have been saying that a file was not found.

Workaround multi-day github actions flakiness:

E: Failed to fetch http://azure.archive.ubuntu.com/ubuntu/pool/universe/l/llvm-toolchain-12/libclang-12-dev_12.0.0-3ubuntu1~20.04.4_amd64.deb  404  Not Found [IP: 52.252.75.106 80]

et al.

Ongoing work making Render() simpler and moving secondary functionality
into separate methods...

(Trying to clean up Render() and make it easier to understand.)