When loading a glTF file on platforms without a filesystem, a client
calls `addResourceData` to populate `ResourceLoader`'s cache with data.
For example, a web client might make HTTP fetch requests to fill in
buffer data. This internal cache of data is stored in
`ResourceLoader::Impl::mUriDataCache`.
This works well, except the cache must persist until after it has been
uploaded to the GPU.
There was already a mechanism (see `uploadUserdata`) in place to ensure
that glTF data persisted until after it had been uploaded to the GPU.
However, this mechanism did not extend to client-provided data. Thus, a
race occured between Filament's driver consuming the buffer and it
getting freed.
44f002a5a1