Add a basic random image generator

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