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.)

This writes spectral images using the layout proposed in "An OpenEXR Layout
for Sepctral Images" by Fichet et al., https://jcgt.org/published/0010/03/01/.

Add `.git` to the double-conversion and zlib submodules URLs.

Issue #226.

Normalize unicode strings for user-supplied names (objects, materials,
media, etc.)  Note that there is no need to normalize strings for things
like the name of the selected sampler, light source types, or the
parameters provided to pbrt objects, as all of the valid ones are plain old
ASCII text. We also intentionally do not normalize pathnames, as doing so
can cause all sorts of trouble.

- #undefine interface, which somehow gets #defined in debug builds, thwarting
  the layered BxDF code
- Move a CHECK out of (possibly) GPU code
- And some cmake tidying

- Break linux/osx/windows into separate .yml files
- Test a smorgasbord of c++ compilers on linux

- Handle "random" -> "independent" sampler
- Rewrite Transform{Begin,End} to non-deprecated Attribute{Begin,End}

- normalMap was missing from most of them
- DiffuseTransmission was missing "scale"
- CoatedConductor was using %f, not %s for printing *Textures.
- etc.

Via issue #223, with thanks to @adruomnfd.

Via @betajippity.

* Check instance defintions for mediums

* Replace structured binding with a traditional iterator

This keeps the loop consistent with the rest of the code base and avoids
creating an unused variable.

Mostly unused variables.

Reading from T * via uint64_t * is not allowed... This led to a significant
slowdown with g++ 9.3.0. (Issue #219).

The computation of the index into the RNG sequence was being done with
32-bit arithmetic (even though the index can be 64 bits), which meant that
it would start repeating after 64k spp.

We still need to consider the glossy case... (Bug introduced in
1f926fd2.)