Scenegraph optimizations

Scenegraph is pretty snappy now, although there are plenty of opportunities left to optimize. Major changes:

* Added `TransformCache` class, which allows `ChainTransform` instances to be re-used across frames. This prevents most of the program recompiling.
* Canvas gets `ndc`, `framebuffer`, and `pixels` entities that are used to represent the coordinate systems it handles.
* Avoid generating throw-away `Variable` instances in `Function.__setitem__`. This was wasting a lot of time.

There is still some cleanup left to do, but once this is merged I think we finally have a usable scenegraph.
Right. So that's why they're public? In general I'd say anything that's public needs a property so it has documentation. But I suppose we can leave that for later in this case.