make crossfade seamless
make crossfade seamless

If the splashscreen happens to look identical to the application, you
can see a short darkening during crossfade, which is quite visible on
bright background.

  * MTexturePixmapPrivate::renderTexture(): blend the splash and the
    application window so that the crossfade (thus the splashscreen)
    is unnoticable in that case.
(cherry picked from commit 94fd90b8a520862d23b173999cfe0ece06102d6c)

nemomobile-graveyard/mcompositor,nemomobile-graveyard/mcompositor,nemomobile-graveyard/mcompositor,nemomobile-graveyard/mcompositor,nemomobile-graveyard/mcompositor,nemomobile-graveyard/mcompositor