pidgin/pidgin

Fix an issue where color_is_visible was incorrectly comparing unsigned values that were potentially wrapping.

Testing Done:
Compile only.

Reviewed at https://reviews.imfreedom.org/r/493/
REVIEWBOARD_URL = "https://reviews.imfreedom.org/"
REPOSITORY = "pidgin/pidgin"
REPOSITORY_TYPE = "mercurial"
BRANCH = "release-2.x.y"
LAND_DEST_BRANCH = "release-2.x.y"
LAND_PUSH = False