This means going back to an immutable renderer reference and interior
mutability for the canvas and the gpu cache. This is because while
traversing the item tree for rendering we may end up destroying
other items due to the lazyness of the models.
This is the start of a different rendering abstraction, the result of a
separate discussion.
The objective is to provide a better backend encapsulation, that will
make it easier to implement other backends and let the backend decide
what to cache and what not. This is done by passing the struct with the
Property<> fields directly to the backend, so that it can decide what
the cache and what to use every time.
