Graphene 1.9.6 (snapshot)

A new snapshot, probably the last towards the 1.10 stable release.

New API and fixes needed by GThree, documentation fixes, and a substantial reorganisation of the repository.

Changes since 1.9.4

• Add radians based initialisers for graphene_euler_t
• Use pragma once for the header inclusion guard
• Remove unused function (#159)
• Add multiply(), scale(), and add() operators to graphene_quaternion_t
• Add lerp() operator to all vector types
• Add graphene_plane_transform()
• Documentation updates for graphene_ray_t
• Fix graphene_box_get_center() and graphene_box_get_bounding_sphere() (#165)
• Ensure that we detect empty or infinite boxes in the graphene_box_t API