Refactor checkRedraw in Canvas

[scheibel]

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.