[interfacekit] last BRegion bug fixed

Hi all.

I just fixed the last known bug in BRegion. The fix is not yet in the CVS (I'll
commit it tomorrow), though I wanted you all to know, since that bug has been
bothering me for some time :)

I could've looked for the culprit code in BRegion for the rest of my life and I
wouldn't have ever found the bug, because it was in one support function in
clipping.h (I can blame only myself anyway).

Thanks to Pahtz for his precious test application, I would've never fixed the
bug without it.

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