A mistake in the mesh normal refactor caused the wrong mesh to
be used when calculating normals with a shape key's deformation.

This commit fixes the normal calculation by making a few changes
in the area.

• The function doesn't make a local copy of the mesh anymore.
• Instead of using the mesh normal API, calculate the normals directly.
• Split the function into three, one for each mesh element.

This required changing a function to apply shape keys to take mesh
vertices instead of the whole mesh. The downside is the lower-level
access to mesh normals, the upside is that they don't have to be copied,
and no temporary mesh is created.