Summary: Adds an abstract base class for random image generation. Implements a concrete subclass for generating Mondrian-like images.
Test Plan:
{F40759}
{F40760}
{F40761}
{F40762}
{F40763}
Reviewers: DeedyDas, AnhNhan
Reviewed By: AnhNhan
CC: chad, aran
Differential Revision: https://secure.phabricator.com/D5705
