Refcounted opaque types should not expose their reference() and destroy() functions

[jackdbd]

As stated in the Cairo Memory Management documentation, reference() and destroy() should not be part of the public API:

Refcounted opaque types all have a ..._reference() function to increase the refcount by one and a ..._destroy() to decrease the refcount by one. These should not be exposed to the user of the language binding, but rather used to implement memory management within the language binding.