writerfilter OOXML tokenizer was just ignoring the mce elements and thus
getting the Choice content and the Fallback one. This initial support
drops all mc:Choice contents to read mc:Fallback. At least for drawingML
vs VML support, we have a much better support of the fallback.

Change-Id: Ic0bf69d0436994e9cfcf38accdd57d17e9f391fe
(cherry picked from commit f4112ce9)