title: GPUImage blend two images with mask image
body: <p>I'm looking to blend two images together using a third masking (black and white) image.</p>
  
  <p>Just like you would do in Photoshop by having two layers and add a mask on the top one, where the mask is white you see the top layer, where it is black you create transparency so the lower layer is visible. Do I need to create a filter with 3 inputs or can I add the black and white image to one of my two images as an alpha?</p>
  
  <p>thanks</p>