title: Using the depth buffer instead of stencil buffer for clipping
body: <p>On older iOS devices the stencil buffer isn't available. Also scissor only works for simple rectangles. For more general clipping can we use the depth buffer? To make things simple, lets assume that we are only drawing in 2D.</p>
  
  <p>Also, my specific requirement is to be able to rotate the clipping rectangle.</p>