To ensure the correct order of calling ForMerge methods,
call flush always before touching maMarkStack.

This was the missing piece in optimizing write() methods,
because of writeBytes() checking each time what to call.
E.g. for Calc documents we don't use maMarkStack at all.
So, just transfer the output to proper "ForMerge" when inside mark()
and allow optimizations.
This commit makes write() methods almost 1/3 as fast.

Change-Id: I96c13888206c81f87e29b998839f78ea9d5570af