Added overdraw visualization.

You may want to make the `registerMutator` function as mentioned below, as this way has a potential bug: if a subclass also sets a mutator for setContext it'll silently overwrite this and break things at run time. If registerMutator asserted that there were no overwrites (the easy 1 line thing to do) that'd be better, then if that happens you could adjust the mutators struct to be a map of lists and have registerMutator chain mutators with the same name. Having the assert would be a good first step.