Const correctness of PackedImageDesc

In our use case, we are passing const void pointers to build a source `PackedImageDesc` for use in `CPUProcessor::apply`. However, `PackedImageDesc` requires a non-const pointer. I assume this is so that users with non-const data can call `getData` and modify the result. This makes it dangerous to const_cast our pointer as a caller could then end up inadvertently modifying a const buffer through those calls.

Perhaps there should be a `ConstPackedImageDesc`? Or is the design to keep the scope of `PackedImageDesc` tightly around the `apply` call so that it's unlikely that users can access the `getData` functions?