Previously cairo-mutex.c was abusing cairo-mutex-private.h by
defining CAIRO_MUTEX_DECLARE before including it, and
cairo-mutex-private.h was simply not overriding any available
CAIRO_MUTEX_DECLARE. This is not the way it should be.
cairo-mutex.c should instead define CAIRO_MUTEX_DECLARE and
include cairo-mutex-list-private.h for itself.
All mutex declarations have been moved to cairo-mutex-list.h.
This should avoid breaking of less frequently tested backends,
when mutexes are introduced or when existing mutexes are renamed.
Instead of initializing mutexes on library startup, mutexes are
lazily initialized within the few entry points of now by calling
CAIRO_MUTEX_INITIALIZE(). Currently only the OS/2 backend takes
care about releasing global mutexes. Therefore there is no counter
part of that macro for finalizing all global mutexes yet - but
as cairo-backend-os2.c shows such a function would be quite
easy to implement.
