Suggest "0.18" as the default base_color

Purely a VFX bias here, but we generally initialise colours to be the value of "middle grey", which as a result of our logarithmic perception, ends up being 0.18.
Was 0.8 chosen because if someone sets metallic to 1.0 with default values elsewhere, then this would result in a plausible metal?

Review the OpenPBR specification and reference implementation to locate the current default for base_color. Compare the rationale for 0.8 with the proposed 0.18 middle-grey value, then determine whether the specification and implementation should change; done requires a maintainer decision on the default.

Written by the indexing model from the issue text.