Merely filling with Qt::transparent is not enough, as the default blend
mode of QPainter is QPainter::CompositionMode_SourceOver, where the alpha
of the source is used to blend the pixel on top of the destination. The
destination in the case of an FBO may contain garbage, and we end up with
the same garbage as the alpha is 0.
This was evident when running the ellipses and porter_duff/porter_duff2
tests on OS X. These tests can now be un-blacklisted.
Change-Id: I315fa764fa29fb3a06e38945a738a6feadf4502d
Reviewed-by: aavit <eirik.aavitsland@theqtcompany.com>
2d1189d9f2