Fix searching for end of inline (EI) images with ASCII85Decode filters (bug 1077808)

Based on http://www.adobe.com/content/dam/Adobe/en/devnet/acrobat/pdfs/PDF32000_2008.pdf#page=33, this is a tentative patch to fix https://bugzilla.mozilla.org/show_bug.cgi?id=1077808.




I've removed this part of the patch, since I'd rather get the main part landed now in order to fix https://bugzilla.mozilla.org/show_bug.cgi?id=1077808 (given that the PR has been open for over two months).
Once this PR is merged, I can submit a followup which changes the inline JPEG image case.