xlib and xcb graphics bindings contain several unused fields by every compatible openxr runtime (Monado and SteamVR)

`XrGraphicsBindingOpenGLXlibKHR` and `XrGraphicsBindingOpenGLXcbKHR` contain the visualid and fbconfig fields, but they are unused in Monado and SteamVR, the only runtimes that are able to use these graphics bindings.

This will probably lead to inconsistent behavior as more runtimes and time goes on, as seen with issues such as wayland client UI toolkits not supporting multiple seats even when the spec allows for it because it was never used in practice, so enforcement or deprecation might be nice.

Start by reviewing the XrGraphicsBindingOpenGLXlibKHR and XrGraphicsBindingOpenGLXcbKHR definitions and checking how Monado and SteamVR use visualid and fbconfig. Determine whether the specification should enforce, remove, or deprecate these fields, and document the chosen behavior for compatible runtimes.

Written by the indexing model from the issue text.