Reduce CPU usage during scanout

[taw10]

Currently, we loop over all fixtures and all attributes, for each one tracing the current value through the state stack. This is very inefficient! It's only necessary to re-check an attribute if it might have changed, e.g. if it's assigned to a function. There are many possible strategies for making the scanout process many times more efficient.

[taw10]

Relates to #1

[taw10]

With 69337c0, it should be a lot better. There is still potential for more big improvements, though.