Better handle SOS CSi values and order

      Image SOS CSi markers should be distinct. Enforce that constraint when
reading multiple CSi values in get_sos().

TBR=darin@chromium.org
BUG=258723

Review URL: https://codereview.chromium.org/27008002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228354 0039d316-1c4b-4281-b951-d872f2087c98