Suggest separate parameters for diffuse and metal colours instead of "base_color"

In the OpenPBR specification, the diffuse albedo and the metallic F0 both share the `base_color` parameter, with `base_metalness` providing the control to mix between the dielectric-base and metal substrates.

We have found it to be useful to have these parameters specified independently as we typically paint these maps separately. Having them shared would make it difficult for us to use OpenPBR.

When Brent Burley chose this parameterisation, his aims were to make a simpler interface for BSDFs, primarily targeted at feature animation pipelines. We have found that for photo-realistic look development, having control of the diffuse separately from the metallic specular gives more artistic control.

Having these colours linked was one of the main barriers that prevented us from adopting Autodesk Standard Surface internally, as well.

Would it be possible to break `base_color` into `metal_color` and `diffuse_color`?

No files, tests, or implementation entry points are named in the issue. Start by reading the OpenPBR specification and the 19-comment discussion around `base_color` and `base_metalness`; done would require an agreed specification change defining separate diffuse and metallic color parameters.

Written by the indexing model from the issue text.