We need this to set data on objects safely without worrying that some
other thread unsets it by setting it at the same time.

Where causing issues for people with MSVC.

Looks *so* much nicer!

Buffer debugging coming soon.

Such that user_data and other finalizers cannot resurrect object

The test/object test is passing again, instead of deadlocking.

For now, by taking a global user_data mutex.

Mutex (and Windows.h by extension) are fairly isolated now.

The Win32 definitions for LONG, ULONG, etc conflicts with
hb-open-type.h.  Avoid that by making sure hb-object-private.h
and hb-open-type.h are not included in the same compilation unit.

Makes way for adding arbitrary user_data support.

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!