QCocoaTheme: Fix size of pixmap returned by fileIconPixmap

The code assumed that CGImageForProposedRect would return a sensible
sized image, but that can return basically any size (depending on the
NSImage's image representations). In the case of the bug report this is
a 1024x1024 pixmap when requesting a pixmap of size 64x64 and larger.
Make sure that we return a pixmap of the exact requested size.
For this, themeHint must also return sizes in device coordinates.

Task-number: QTBUG-35009

Change-Id: Iaae11023bab6d4122815ca4010aab6967dfb18a0
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>