Add dirty detection and fix a bug in SpriteSheet

Summary:

• I had some W vs H mixed up, fix that.
• Allow sheets to detect when they need to be regenerated (any source file, rule, dimension, or position has changed) so we don't need to do extra work.

Test Plan: Generated sheets with non-square icons without hitting layout bugs. Ran sprite generation and saw only dirty sheets be picked up for regen.

Reviewers: chad

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D4031