The previous code resulted in that all regions that had some alpha
blending content had to be rasterized as bitmaps.
With this patch, we only need to fallback to proper alpha blending
if there is a background content to blend against.
If there is no background content we simly assume that the background
is white (which happen to be the most common paper color). It will
then be treated as a solid background.
Task-number: QTBUG-33548
Change-Id: I9d2c9be95a701704cdb3724480421db49b4cdd98
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>
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