Happens when div loading is dependent on resources loading.

- Instead of modifying the height of the body (and html) after render we
  just set the mask to take care of it. The image we were getting was of
  the full size page cropped down to the height and the rest filled with
  white. I think this is because the larger initial page size caused the
  viewport to expand but then it didn’t contract.

- Since args.div will supersede the mask eventually, don’t set the mask
  up front if args.div is present. Set the mask solely on the height arg
  as we can default top and left to 0.

This fixes the failing unit tests when running locally.