The title says it all actually.
Added special custom data type, because we don't know in advance
whether we're referencing UV or Color layer. Also made it so vertex
attributes are normalized.
TODO: Border render in viewport ignores the normalization of the
attribute array for some reason, will be looked into still.
Reviewers: mont29, brecht, campbellbarton
Reviewed By: brecht, campbellbarton
Differential Revision: https://developer.blender.org/D2022
a830280688