If present, the shading normals should determine the orientation there,
as they do with intersections.

Rework interfaces for RealisticCamera exit pupil sampling and
providing the PixelSensor illuminant.

Fixes GPU build.

Via @pierremoreau. Fixes issue #140.

A few function renamings.

Beyond the convenience of having them as a vector of strings, this
is where UTF16 -> UTF8 conversion of args happens on Windows.

This led to some adjustments to ParseArg()...

With this (and a number of preceeding commits), #136 should now be fixed.

This should make UTF8 filenames work with the image formats handled by
stb_image on Windows...

Put cpu/ source files in their own subfolder for MSVC
Add the cpu/*.h files, which weren't included in PBRT_SOURCE_HEADERS before.

Fixes a few "host function called from device code" warnings on Windows with the GPU build.

...and fix assorted implications.

The conversion of filenames from UTF8 to UTF16 for Windows is then handled here.

Fortunately, the compiler error was fixed in 11.3.1...

(Issue #124.)

Issue #136.

(No more need for DispatchCRef, etc.)

This effectively requires handling each number of types stored by
TaggedPointers with a customized dispatch function(*), but it has the
advantage that stack traces when debugging only have one level for dynamic
dispatch, rather than log2(n) levels of the binary search over indices to
step through as before.

Performance is unchanged and perhaps marginally better.

Fix an edge case where a sample could be returned from the last bucket even
if it had a weight of 0. Also, divide less.