title: EdgeCollider2D for a ParticleSystem in Unity3d?
body: <p>I am just stuck at a very simple problem i guess :)
  I want to draw a edge collider like the black line show in the picture below. The Problem is that i only can move the collider to the left or the right, but not up or down. Is there a shortcut or something that can help me??</p>
  
  <p><img src="https://i.stack.imgur.com/L9E4G.jpg" alt="enter image description here"></p>
  
  <p>Thank you very much in advance!</p>