Revert "Avoid type checking overhead for resize-mode"

Revert "Avoid type checking overhead for resize-mode"

This reverts commit 3eacfa88f2d9c690a006de9cac1cd3c60901686c.

Apart from the patch not being correct, we don't want to expose private
structures in header files if we can avoid it.
And this type-checking overhead is not an optimization that is even
measurable.

https://bugzilla.gnome.org/show_bug.cgi?id=754932