... attributes that happen if both CharHighlight and CharBackColor
properties are used, because the CharBackTransparent property wasn't
taken into account, and combining the CharBackColor and
CharBackTransparent properties happens *after*
XMLTextExportPropertySetMapper::ContextFilter() runs.

Also, it looks like a transparent highlight wouldn't export properly but
apparently DomainMapper::getColorFromId() won't create such.

(regression from f880962f)

Change-Id: Ib628ef8bb377482f74fadb97c81afb95fbbf7184