RECT pascal RectIntersect(RECT r1, RECT r2)

  This function produces the intersection of two rectangles. The
intersection is the largest rectangle which is contained in both
rectangles. The resulting rectangle is returned.

Parameters
  r1 and r2 are passed rectangles.

Returns
  A rectangle whose coordinates represent the intersection. If there
is no intersection, an empty rectangle will be returned (which can
then be tested by RectIsEmpty()).