Fix memory corruption in shader materials.

Change ee616b3905106a3eedef9ee964ab283ef45c7dbc accidentally
removed the member variable used to refcount the attribute
names. This resulted in the names being deleted and the
stored const char *'s to become invalid. Add it back.

Change-Id: Ie33f75cd76085283a5ee685602e023bb3c42c896
Reviewed-by: Robin Burchell <robin.burchell@viroteck.net>