Add graphene_box_init_from_vectors
graphene_sphere_t: Add getters for center and radius
Add graphene_sphere_init_from_vectors

This is needed for GThree where we have existing meshes with
vec3 vertices that we need a bounding sphere for.
Add graphene_matrix_transform_sphere

Transform a sphere to a bounding sphere for the transformed sphere.
Very useful for frustum culling.
Merge pull request #27 from alexlarsson/master

Add various things that are needed for GThree