Fix T95334: Crash with no vertex normals in multires bake

This is partially caused by a stupid mistake in cfa53e0fbeed7178c78
where I missed initializing the `vert_normals` pointer in
`MResolvePixelData`. It's also caused by questionable assumptions
from DerivedMesh code that vertex normals would be valid.

The fix used here is to create a temporary mesh with the data necessary
to compute vertex normals, and ensure them here. This is used because
normal calculation is only implemented for `Mesh` and edit mesh, not
`DerivedMesh`. While this might not be great for performance, it's
potentially aligned with future refactoring of this code to remove
`DerivedMesh` completely. Since this is one of the last places the data
structure is used, that would be a great improvement.

Differential Revision: https://developer.blender.org/D13960

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