Fix graphene_ray_transform

[alexlarsson]

We can't use graphene_matrix_transform_vec3() to transform the origin
because we need to point to respect transformations (but not the
directions, so we can use vec3 there instead of reconstructing
the direction).

Fixes #...

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[alexlarsson]

I added support for raycasting to gthree HEAD (see examples/interactive), and without this its totally broken. With this fix it mostl works although sometimes it seems something goes wrong and nothing is selected.

Also, part of the support was adding a bunch of ray-to-$primitive helpers, and these should probably be in graphene: https://github.com/alexlarsson/gthree/blob/master/gthree/gthreemesh.c#L10