This lets us generate long lists of settings permutations by writing a
simple JSON spec. For example, the following spec would generate six
`Settings` objects, all of which have SSAO enabled. See the unit test
for a larger example.
```
{
"name": "viewopts",
"base": { "view.ssao.enabled": true }
"permute": {
"view.dithering": ["NONE", "TEMPORAL"],
"view.sampleCount": [1, 4, 8]
}
}
```
This introduces the `viewer::Settings` struct, and a JSON reader /
writer.
This will be used for automated testing and for client / server
communication.
Note that `viewer::Settings` is closely associated with the
`filament::View` API; when updating the latter we will often need to
update the former, as well as some serialization code. This increases
the maintenance burden and I think we should consider using a parser
library like libclang or a macro-based reflection utility.
This PR also migrates SimpleViewer into libs/viewer and un-inlines its
implementation. It does not belong in gltfio because it has an imgui
dependency.
