* 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.

(Rather than just printing a warning.) Issue #152.

Now Material implementations have a GetBxDF() method rather than GetBSDF()
and they directly return their associated BxDF type, whatever it may be. This
slightly cleans up all of the GetBxDF() method implementations with no real
impact on the code that calls them.

Fixes #179, via @shadeops.

Partially seeding an RNG using just ray direction is risky business with an
orthographic camera. Things go extra bad when the object enclosing the
medium has camera-axis aligned planes in which case doing the rest of the
RNG seeding with tMax to the intersection point rounds out the borkage.

Fixes #175.

s/BumpEvalContext/NormalBumpEvalContext/
Factor Bump() into BumpMap() and NormalMap()

Rename texture mapping classes to not have "2D" and "3D" suffixes.
Otherwise just naming and formatting changes.

Just variable renaming and formatting...

The Camera, Filter, Film, and Sampler are all created using RunAsync() calls as
soon as their specifications are available. This also cleans things up from the
caller side as we take care of the dependencies between them (e.g. the Sampler
needing the Film resolution) once inside BasicScene.

Rather, check the actual objects used for rendering. (This will allow making more
members of ParsedScene private...)

SceneStateManager -> BasicSceneBuilder
Add various object creation methods to BasicScene
Remove redundant threadAllocators parameter from BasicScene::CreateMaterials()

Remove SceneProcessor interface class (only had one implementation)
Rename ParsedScene -> BasicScene

(Following a02c2d0c.)

This includes two minor bugfixes related to proportional PDFs from BxDFs and
volumetric emission that also eliminate consistencies with the VolPathIntegrator.

s/T_hat/beta/g
s/uniPathPDF/inv_w_u/g
s/lightPathPDF/inv_w_l/g

In preparation for changes corresponding to a02c2d0c. Note that
the variable names do not match their actual usage, but this makes the forthcoming
actual logic changes easier to review.
logic changes

Make SquareMatrix::Determinant() a stand-alone function.
Remove rarely-used Bilerp() function

Issue #172.

Variables were removed or renamed in 59b5988d and 24c6ef53

Issue #170.

Qualify Medium::IsEmissive() with PBRT_CPU_GPU

Get rid of the MediumSample structure; pass the various appropriate values
directly to the lambda passed to SampleT_maj() rather than wrapping them
up into that.

Simplify MediumInteraction to not include sigma_* and Le (it's not necessary
and just causes a bunch of data copying.)

Make SampleT_maj a standalone function
Otherwise minor variable renaming and restructuring.