neraulgraphics 0.3.0

Triangle meshes and dynamic scenes.

Added

• Apache-2.0 license.
• Triangle meshes. Mesh is geometry, built into a BVH once; Instance places a
  mesh with its own Transform and Material. The number of instances is not
  capped, unlike spheres. Pass them to Renderer(..., meshes, instances).
• Renderer::set_instance_transforms() (Python: an (N, 3, 4) array or a list of
  Transform) moves instances without touching the meshes. The tree over
  instances is refitted in place and rebuilt only when it has loosened too far.
• make_transform (position, (x, y, z, w) quaternion, scale), make_box and
  make_icosphere, in C++ and Python.
• make raytracer builds the viewer and links it as ./raytracer; make run
  and make test do the obvious. The viewer takes --instances N to spawn
  spinning shapes.

Known limits

• Triangles are shaded flat, with no vertex normals or textures.
• Updating instances runs on the CPU: about 9 ms for 10,000 and 90 ms for 100,000.
• Ray-triangle tests are not watertight along an exact shared edge.