Various fixes to rendernode drawing with Cairo

This is fallout from me looking at the clip-removal from #2431 (closed) and !1418 (merged) and contains bugfixes (gdk_cairo_get_clip_rectangle() removal, testsuite refcounting and gsk_rounded_rect_intersects_rect()) as well as refactorings (gsk_cairo_rectangle(), the blur stuff).

Not sure if all of the patches are desired, so I'll leave it to @baedert to give final approval.