08a8de739d
`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