The default padding I introduced in d4f81fb4e6 is
actually only 4 pointers and on 32bit platforms already smaller than the union.
Replace it with a fixed 64byte padding. Don't add the normal padding for now.
Fixes#755822
While this technically is an abi break, we decided to do this:
1) the struct is documented to be internal
2) the struct is alloced and freed inside the library
3) there are no public methods that receive or return instances
4) the only code known to use this struct are classes containd here
