QTiffHandler: move some private methods to the Private class

This is in preparation of an important read() robustness refactoring
(dragging the error-prone manual d->close() out of the functional
parts of read()).

As a drive-by, drop the floatingPoint argument of rgb48fixup(),
which would otherwise now shadow Private::floatingPoint(), and which
the method can just as well read from *this.

Pick-to: 6.12 6.11 6.8
Change-Id: I3d64ce4cf7e34c5b51674ca16c601e9a4c393d2b
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>