Add support for using g_autoptr with graphene types

[tintou]

Proposed changes:

• add g_autoptr(graphene_$TYPE_t) support

This is handy when writing an application.

[ebassi]

To be honest, if you're writing a C application I'd recommend to use Graphene types on the stack instead of using the heap; using g_autoptr works only if you're copying data from out of properties or signals.

[tintou]

Yes.

In my use-case I have a GList of points so I wanted to use a g_autolist(graphene_point_t) my_list;