fdo#54361: Chart background in XLSX is transparent instead of white.

Conflicts:
	oox/source/drawingml/chart/objectformatter.cxx

Change-Id: I116e7e8c9046009cfcadc04b5367a6fe25f88d96
Reviewed-on: https://gerrit.libreoffice.org/11910
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>