* 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.)

It seems that we see a slowdown with multiple GPUs even with the same
shader model. (Issue #217.) Therefore, now pbrt issues an error if multiple
GPUs are detected and one hasn't been selected.

Fixes #216, via @harrytodorov.

Fixes #214; issue noted by @anooprthomas.

(And reenable the unit test that confirms this.)

It's wasteful to expand the provided Float to an Interval since then
redundant work is done in the full (Interval op interval) implementation.

This gives a ~5% speedup for scenes with many quadric shapes, where
Interval is used heavily in the intersection routines.

Silences a compiler warning with nvcc 11.5.

(The MSVC build fails when these are inconsistent.)

* Add rendercoordsys as a scene Option

* Remove quote protection as it is not needed

This works around a NaN case in SampleSphericalTriangle(). (Ideally that
function would be more numerically robust, but that's a bigger issue...)

Issue #211.

Via @shadeops. Issue #210.

A missing ResolveFilename() in the function that associated loaded normal
map images with materials caused normal maps to be dropped when pbrt was
started in a different directory than the scene file was in.

Fixes issue #210.

Due to round-off, BasicSceneBuilder::CTMIsAnimated() may return true even
though the corresponding AnimatedTransform ends up storing the same two
transforms. This was triggering an assert in the AnimatedPrimitive
constructor...

Fixes #204.

Accessing parameters of the std::moved-from entity was not effective...

Null-valued member variables were leading to trying to initialize std::strings
with nullptr rather than printing "nullptr".

Reduce the epsilon to better resolve fine shadows (especially in the Moana Island scene.)

Also add a hack to fix a rare infinite loop when finding all intersections
along a ray for subsurface scattering with quadric shapes (root cause bug
not yet chased down.)