Talking to Ryan Lortie, he thinks my comment doesn't make sense.

So I'm making the getter const.  Note that g_atomic_int_get()
casts that away itself, so we don't need to worry about that
(which kinda makes me uncomfortable actually).

So we can liberally use the simple features of C++ that parts of the
codebase is already using.

This was a bizzare piece of API that I inherited from cairo.  It has
been wrong adding them to cairo in the first place.  Remove them before
someone uses them!

Such that we don't break debug build all the time.

Caught by John Daggett.

Not used yet.

This reverts commit c939f6aff405ca7b10b1f1538f46148bff719fcb.