title: Apply a mask to a UIView to simulate a glass shimmer
body: <p>My base image is a complex shape with multiple colors and an alpha background. </p>
  
  <p>My shimmer effect is a white slanted gradient with an alpha background.</p>
  
  <p>My goal is to animate the shimmer image over top of the base image, but make the alpha values of of the shimmer match those of the base image.</p>
  
  <p>I've only used layer masks to hide areas based on the color, not to both show AND hide parts of an image. Will I need to create multiple masks to achieve this? A white version of my base image that will show through the masked shimmer image, all on top of my colored base image?</p>