The problem is that if the FillStyle is NONE, setting only
ParaBackTransparent but not ParaBackColor does absolutely nothing.

This means that if a parent style sets a background color, then a
derived style cannot override it to be transparent.

Cause is that getSvxBrushItemFromSourceSet() defaults to setting a
0xFF transparency on the SvxBrushItem.

There is still a problem that getSvxBrushItemFromSourceSet() sets the
pool default item's color and that is also exported to ODF again as
draw:fill-color, but together with draw:fill="none" it should not cause
harm, at least for consumers that prefer draw:fill attributes like
LO >= 4.4.

(regression from 7d9bb549)

Change-Id: I5312901166a0e6f0cf02369b173dc8498e4113ed