gallium/util: return FALSE for intersection if there's empty rectangles

gallium/util: return FALSE for intersection if there's empty rectangles

It isn't really obvious if intersection test should take into account empty
rectangles or if the caller should do it. But it looks like most callers
actually verified one of the rects but not the other, but since correctly
returning an empty rect that other rect could actually be empty leading to
more bugs. Hence just verify both rects for emptyness in the intersection
test itself which makes the code easier in the caller (though it will be
slower if the caller knows the rectangles are non-empty).

Reviewed-by: Zack Rusin <zackr@vmware.com>