Fix transparent toplevels on Mac OS X again..

We're using QWindow::format() to decide opacity or not in
a few places, but this used to resolve to
QPlatformFormat::format() which would in turn return a default
format without alpha set. Instead, return the format requested
by the user.

Masked windows were always broken as converting a 32-bit image
to an Indexed8, doesn't give a grayscale image, but rather
a randomly spreadout set of indices based on the colortable
generated by the converToFormat function.

Task-number: QTBUG-28531
Change-Id: I537288f85c70b1e6194785b9ebcb5ea1f9581cee
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>