Since the text layout is being redone then the visibleImgTags will be
repopulated, therefore it should be cleared first so that it doesn't have
duplicated entries.

Change-Id: I415f94326e156ae265bc044544f2bb0d7146ce4c
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>