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