Graphene, part 1

This is the first step at moving to Graphene. This MR is organized as:

1. Various cleanups
2. Add Graphene dependency
3. Port various data structures to Graphene

Namely, the types that could be replaced by Graphene counterparts were:

• ClutterGeometry → graphene_rect_t
• ClutterPoint → graphene_point_t
• ClutterRect → graphene_rect_t
• ClutterSize → graphene_size_t
• ClutterVertex → graphene_size_t
• ClutterVector* → graphene_vec*_t
• ClutterVertex → graphene_point3d_t
• CoglEuler → graphene_euler_t
• CoglQuaternion → graphene_quaternion_t

This merge request does not replace CoglMatrix by graphene_matrix_t though. The semantic differences between those two are massive, and after literally dozens of failed experiments on that front, I'm convinced that CoglMatrix will require a merge request of its own. It's simply too massive.

To Do

• Add various APIs to graphene (!141, !149)