vela: Change Shape4D mutability usage

 - Removed requirement for cloning shapes when unique values required
   by forcing top-level immutability. This alleviates issues with Shapes
   being unintentionally shared and then mutated as if value-types.
 - Shape4D fields can no longer be assigned without replication.

Signed-off-by: Tim Hall <tim.hall@arm.com>
Change-Id: Ic0dbfa349eb0215eabefb4f4e2cf99f12d83699c