i.e., match the r_u/r_l notation used in the book and in
VolPathIntegrator.

Enable fullscreen rendering.

Fix crash on Wayland.

Avoid segfault if XDG_SESSION_TYPE is unset

Now the event loop continues running so that the camera can be moved
further, etc., though we don't take more samples than specified since many
samplers give unexpected / not useful results in that case.

Issue #299.

Fix denoise issues

This patch fix a issue using denoise because a Variance channel names exposed here: https://github.com/mmp/pbrt-v4/issues/300

Issue #292.

Fix nullptr access reported in issue #293

Corrected denoiseAlpha parameter type

The previous implementation would inadvertently reuse the same sample value
for medium distance sampling when rays passed through interfaces between
different scattering media. In turn, this led to significant errors in
rendered images.

See issue #221...

There's no need for the user to see a fatal error, which should be reserved
for an actual internal error rather than a problem with the scene as
specified...

Issue #279.

Add a option to use arbitrary names for grids

Sometimes .nvdb files use arbitrary names for grids. This change adds two options for the user to specify different names for 'density' and 'temperature' grids.

Previously, we'd get NaN values with anisotropic surfaces with zero
roughness in one direction and non-zero roughness in the other.

Fixes issue #283.

Avoid Clang 14 error compiling under Windows

Check to see if a filepath exists before making absolute

Build with Clang 14 under Windows show this error
```
clang-14: error: no such file or directory: '/D'
clang-14: error: no such file or directory: 'PTEX_STATIC'
```
This commit fix this and is also OK for MSVC

Only resolve aperture as a file if not a builtin name