[interfacekit] integer_rect

I noticed Darkwyrm committed a change (for Caz) which adds a integer rect,
well, we have already such a structure, it's called clipping_rect, and we also
have some functions (in clipping.h, in the private/interface) folder, which can
handle those rects. I think it would be better to use that struct instead of
creating a new one, right ? :P

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