Support optional caching of rotated shapes to disk

[tmontes]

Facts

• Bitmap shapes support rotation.
• Rotated variations of the source image are computed.
• Such computation is not fast, especially when no 3rd-party libraries are installed.
• They are cached in RAM for the lifetime of the running process.
• New processes have to recompute everything: potentially slow and wasteful.

Thoughts

• Adding support for optional caching of such computed images to disk may be valuable for some use cases.

Implementation idea

• The shape could be given a "cache object factory" callable, defaulting to dict.
• Such factory would create a mutable mapping object that the shape.Base class would use.

This could be enough, I suppose. With this, simple disk-based caching could be achieved by passing one of the classes in the shelve Standard Library, for example.

Extra

• One or more custom mutable mappings and their factories could be included.
• One such custom mapping could even support having everything bundled with the code itself: no external disk-based dependencies.
• An alternative, very easy to use API could be made available too:
  • Maybe the above mentioned "cache object factory" would be passed to a cache shape argument.
  • When given a callable, the implementation above would be used.
  • If given a str of path.Path it could take that as the file to be used for caching, taking care of the dirty work of setting and wiring it up. :)