static Rect intersectRects(Rect a, Rect b) {
  int x = max(a.x, b.x), y = max(a.y, b.y);
  int x2 = min(a.x+a.w, b.x+b.w), y2 = min(a.y+a.h, b.y+b.h);
  ret new Rect(x, y, x2-x, y2-y);
}

static Rect intersectRects(Rect a, int x1, int y1, int w, int h) {
  if (a == null
    || a.x >= x1 && a.y >= y1 && a.x2() < x1+w && a.y2() < y1+h)
    ret a;
  ret rectFromPoints(
    max(a.x, x1), max(a.y, y1),
    min(a.x2(), x1+w), min(a.y2(), y1+h));
}

Began life as a copy of #1005838