RECT pascal RectUnion(RECT r1, RECT r2)

  This function produces the union of two rectangles. The union is the
smallest rectangle which is contains both source rectangles. The
resulting rectangle is returned.

Parameters
  r1 and r2 are passed rectangles.

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