GCDAsyncImage

Challenge: Fix this project using GCD.

Implement the following:

  1. Increasing scrolling FPS of images
  2. Implement task cancelling for images that no longer need to be download from the internet
  3. Apply image filter
  4. Implement task cancelling for images that no longer need to be processed for filtering

Requirement: For the purposes of learning GCD, you must download image data with the Data(contentsOf:) API.