Fix backdrop color

Summary: All our builtin images use #c4cde0 for the backdrop. This makes generation match the builtins.

Test Plan:
Build a new bug icon in Maniphest

{F1077934}

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D15126