Add `rectIntersection` to rect.um/coll.um

[skejeton]

returns intersection rect between two rects.

fn rectIntersection*(a, b: rect::Rect): rect::Rect {
  x := max(a.x, b.x)
  y := max(a.y, b.y)
  return {x, y, min(a.x + a.w, b.x + b.w) - x, min(a.y + a.h, b.y + b.h) - y}
}

[marekmaskarinec]

Make a PR

[skejeton]

I'm saving this for after we're done with the API update.