`MPoly` is used and copied in many places. To avoid the need to use a
special function for copying MPoly, or the need to add a copy
constructor, just rename the `mat_nr` field to include "legacy" in the
name. This keeps the original purpose of notifying developers that
the field shouldn't be used without any further complexity.
Apply the same fix to `bweight`.
Differential Revision: https://developer.blender.org/D15841
08a8de739d