Make ch3_panorama at least 8x as fast.

Since you're accepting pull requests, here's another one :-)

Your ch3_panorama.py took more than 8 minutes to run (after which I killed it). My version runs in 3 seconds. This was by far the biggest win.

If you're interested in further speedups, feel free to look at https://github.com/nico/cvbook/commits/master (https://github.com/nico/cvbook/commit/349b23664cbece5bab512f7a07110f877bd4e159 and https://github.com/nico/cvbook/commit/31a56c3963e15657b9d3f14ed91530e0f683ec8a are simple and effective for example.