Fix calculation of spritesheet hashes to use relative paths

Summary:
When a user generates sprites, we hash all the inputs to figure out what we actually need to regenerate. Normally (i.e., on my machine), this works fine.

However, part of the hash is the absolute path of all the input images, so my machine hashes in `/Users/epriestley/phabricator/path/to/sprite.png`, and then @chad regens them and gets a hash of `/Users/notchad/phabricator/path/to/sprite.png` instead, or whatever. The hash comes out different so every manifest and sheet gets regenerated.

Instead, hash the //relative// path -- e.g., `path/to/sprite.png`. This keeps the path the same across machines and means that only sprites which have actually changed will regenerate.

See also next diff.

Test Plan:
  - Before this patch: generated sprites in `phabricator/`, then moved `phabricator/` to `phabricator2/` and regenerated. Incorrectly, all sprites were regenerated.  - After this patch: generated sprites in `phabricator/`, then moved `phabricator/` to `phabricator2/` and regenerated. Nothing was regenerated.

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D5964