img_auth: handle files in the transcoded zone

img_auth.php does not currently handle filepaths for transcoded videos
at all, and instead falls back attempting to open files out of the
public zone, which obviously fails as these files do not exist there.

Add a branch that handles transcoded containers in the same manner as
thumbs and thus allowing access to those files from private wiki.

Bug: 65007
Change-Id: I7182206ba09dd2466fc51aa1d51116dd13db980c