Add a lot more sanity checks to region code

Add a lot more sanity checks to region code

- Introduce a GOOD_RECT() macro that checks that a pixman_box_t is not
  empty or degenerate an use it.
- Use GOOD_RECT() instead of magic if statements for funtions that take
  x, y, width, height arguments
- Use GOOD_RECT() in _reset(). The checks in the previous code seemed to
  allow an empty box, but then created a broken region from it.
- Add GOOD(region) check at the end of _translate()