(Tests) Replace grabFrameBuffer with renderPixmap().toImage()

There is no warranty that grabFrameBuffer returns a scene fully
rendered, whereas renderPixmap will always render. Without this
commit, on Windows, result images are either partially or
completely black.
(GraphicTest) Wait a little bit before testing for paint event

Without this, the paint event was not received on Windows.
Merge pull request #209 from tlecomte/tlWindowsGraphicTests

Tl windows graphic tests