Refactor checkRedraw in Canvas

Currently, checkRedraw iterates over *all* (actually, only some) outputs of the render stage and triggers a redraw upon first invalid output.
Instead of reiteration, the output invalid state can be set upon the firing of an invalidation signal of either render stage output.
However, when registering to output invalidation signals, adding and removing outputs over the lifetime of a render stage has to be handled, too.

Start by locating Canvas::checkRedraw and the render-stage output invalidation signal handling. Trace how render-stage outputs are added and removed, then ensure invalidation state is updated through those signals for the full output lifetime; done means redraw decisions no longer reiterate outputs and dynamic output changes remain handled.

Written by the indexing model from the issue text.