Fix: graphene_box_expand_vec3

We want to use min/max, not subtract/add, just like the normal
graphene_box_expand.
Fix graphene_plane_normalize()

This didn't take the current constant value into account, breaking
if res != p.
Fix graphene_matrix_transform_sphere()

Actually apply the matrix to the sphere center.
Merge pull request #30 from alexlarsson/master

Fix various issues that break frustum clipping in Gthree