PHUIImageMask

Summary: Adds a PHUI class for display images on a center point, with or without a mask.

Test Plan:
I am bad a math, so like, check that for me please. I tested using Photoshop. Class may need tweaked depending how we store the inline-comment coords.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9614