On certain linux, macOS environment, there is already a system
getopt. This often creates conflict when compiling filament.
Here we alias utils::getopt to either the system getopt (if
present) or third_party/getopt.
Fixes#7551
- spectral reconstructions with 4 samples
- unroll the whole dispersion computation to improve performance
(batch texture fetches and reuse common values)
- tool to generate the matrices for spectral integration