Save opaque images as opaque TIFF files

We currently save all non-indexed images as RGBA TIFF files. We should
save them without an alpha channel, to save space and maintain the
information that they are opaque.

Task-number: QTBUG-18475
Change-Id: Id656f4078ea0a1b88235fb04add99a4680422354
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Ivan Komissarov <ABBAPOH@gmail.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>