core: GJKSolver is a struct

src/distance_box_halfspace.cpp

src/distance_box_plane.cpp

src/distance_box_sphere.cpp

src/distance_capsule_halfspace.cpp

src/distance_capsule_plane.cpp

src/distance_cone_halfspace.cpp

src/distance_cone_plane.cpp

src/distance_cylinder_halfspace.cpp

src/distance_cylinder_plane.cpp

src/distance_sphere_cylinder.cpp

src/distance_sphere_halfspace.cpp

src/distance_sphere_plane.cpp