Use RGBA8888 format to simplify TIFF writing

We write TIFF files using big-endian RGBA, by using the RGBA8888 QImage
format we can remove the manual conversion done in qtiffhandler.

Change-Id: I4a7f4810ce4332d1608813d9cd6371bc13d94df0
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Ivan Komissarov <ABBAPOH@gmail.com>
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>