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.
The most recently loaded model is now automatically re-loaded if you
re-start the sampler-gltf-viewer app.
Super useful in my case for `rgb-lights.glb` :)
This adds support for decoding HDR files, dropping them into the remote
web page, and sending them over the wire. However the decoded results
are not yet passed into IBLPrefilterContext, that will be subsequent
PR.
Rename KtxLoader to KTXLoader.
The remote UI has been tweaked a bit, so that it shows a more helpful
message when disconnected.
