* Add multi-scene support to gltf_viewer.
To test this, I generated a multi-scene asset as follows.
```
gltf-transform merge Avocado/glTF/Avocado.gltf \
BarramundiFish/glTF/BarramundiFish.gltf \
~/Desktop/Merged.gltf
```
* Specify c++17 in pbxproj files.
When the guard band is enabled, the effective rendering area is
increased by a certain amount (currently 16 pixels on each side) so
that screen-space effects (e.g. SSAO, SSR, DoF) behave better around
the edges of the screen. Of course, this comes at a performance and
memory cost.
Currently the guard band is not configurable other than being optional.
Added c++, java and js bindings.
- For completion, FilamentAsset now has getRenderableEntities(). This is
similar to sister methods getLightEntities() and getCameraEntities()
except there is no need to store a separate array.
- The web helmet demo does not need to enable shadows, they are already
enabled.
- The ViewerGui populateAsset() method was doing two things that are
now decoupled for clarity: setAsset() and populateAsset().
- The updateRootTransform() method is now called only when the autoscale
checkbox is toggled, instead of every frame.
- The getFooEntities() methods in Java now skip doing work for empty
lists. Actually these should not return arrays at all, but let's fix
that later, since it will break backwards compatibility.
The SimpleViewer C++ class was not viewer component like `ModelViewer`
and `<filament-viewer>`. It was actually just the UI builder used
in the `gltf_viewer` desktop app and the remote Android interface.
