widgetpaintable: Protect against too many signals

widgetpaintable: Protect against too many signals

This is actually not just a mechnaism to protect against too many
signals, but it's also a method to getting those signals at the wrong
time.

For every size/content change, a widget needs to invalidate twice:
Once when it queues a resize/redraw (going valid => invalid) and once
when the new size/content is actually assigned (going invalid => valid).

However, one of those invalidations might be inconvenient for the
listener. GtkImage for example does not like receiving
invalidate-contents signals when new contents are assigned, but is fine
with them when the old ones go invalid. And it will not try to draw the
paintable in between anyway.

So by bypassing the 2nd emission if nothing was changed, we can make
GtkImage happy.