It was impossible for drivers to use shape key properties, modifiers
generate a new mesh. After mesh evaluation the shape keys are no longer
necessary, and because of this the `key` pointer was not copied. As
drivers work on evaluated data, however, they do need this `key`
pointer.
This commit makes the `key` pointer available in evaluated meshes, but
this is somewhat dangerous. There was an explicit reason why the key on
result was kept at null pointer: to have the evaluated mesh in a
consistent state. Assigning this pointer makes it potentially
inconsistent, as the evaluated mesh and the original shape key may have
different topologies.
Reviewed By: sergey
Differential Revision: https://developer.blender.org/D7785
cb2565195e