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.