title: How do I provide offsets for the output texture or framebuffer in WebGL?
body: <p>If I want to use a certain region in my input texture and not the entire thing, I could use <code>gl.texSubImage2D()</code> and provide <code>x</code> and <code>y</code> offsets. </p>
  
  <p>What would be the equivalent in the output texture? Given that in my fragment shader I do not have control of what texels i'm writing to.
  Would a call to <code>gl.viewPort()</code> do the trick? Do I need to change canvas dimensions for that?</p>