bnc#886540: Default chart background for pptx docs should be transparent.

Charts in docx and xlsx OTOH use solid white as the default fill style.

Change-Id: Ic4351fe65cabc12d60214b67c7026a317841f2c7
bnc#886540: Add test cases for chart background fill variance.

"Automatic" fill in docx and xlsx should be solid white, whereas in
pptx it is no fill (transparent).

Change-Id: I7e64f2f895e4c4ed072a234e622a348599456f37