This was only used in a single place, multires baking, where it's simple
to just retrieve the looptris from the temporary `Mesh` that we already
create to simplify accessing normals. This allows removing a bunch of
complexity from `DerivedMesh`, to help progress towards its complete
removal at some point in the future.
Also use `Span` more instead of raw pointers in multires baking,
and pass around the spans instead of the low-resolution DerivedMesh.
7adea7ee15