Fix incorrect MultiSphereConvexHull rendering

[jslee02]

Previously, MultiSphereConvexHull was rendered as a composition of spheres and cylinders (that connect the spheres) rather than the actual convex hull. To fix the issue, this PR adds Mesh, TriMesh, and Icosphere classes, where they provide a feature of convex hull generation. The GLUT and OSG rendereres now draw a convex hull for MultiSphereConvexHull by (1) generating icosphere meshes from the spheres, (2) merge then as a single triangle mesh, and (3) lastly generating a convex hull out of the merged triangle mesh.

Also, this PR is a preliminary work to address #453.

Old:

New:

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