plane clipping, is it possible ?

[mulle-nat]

If I have a plane on top of another plane, the planes rectangles on drawing don't intersect to clip the drawing area. The only sort of "clipping" operation, that I can find in notcurses is the merge down. Is this right ? I experimented with changing the rectangle given to paint, but the result looked all garbled. This problem is sorta critical to me and ties in with #2761 I feel.