`windows-rdl` doesn't quite handle `...Gdiplus.Color`

This struct has a bunch of scoped constants that look like enum variants but are really const fields in metadata. We need to find a suitable way to represent those in RDL.

Start by locating the windows-rdl handling for metadata-backed constants and inspect the representation of Gdiplus.Color. Determine how its scoped const fields should appear in RDL, then verify the resulting representation against the struct's metadata and generated output.

Written by the indexing model from the issue text.