Use "-dispose background" to improve reassembly of GIFs with transparency

Summary:
Fixes T5741. We break GIFs apart with "-coalesce" which completely rasterizes each frame, but stitch them back together without specifying "-dispose".

This produces the default "-dispose none" behavior, which causes GIF frames to "pile up" if they contain transparency.

Instead, use "-dispose background" so that the previous frame is erased before each new frame is drawn.

Test Plan: See T5741 for additional details.

Maniphest Tasks: T5741

Differential Revision: https://secure.phabricator.com/D19214