macOS: Don’t color convert the backing store

The backing store was assigned the sRGB color profile
as an unintended side effect of the QImage -> CGImage
conversion function refactoring in ac899f6d

. This
caused Core Graphics to add a color convert step, which
in some cases caused performance issues.

Restore fast, previous behavior by assigning the target
display color profile to the backing store image.

Color correctness is still a goal, but we’ll add API
for it and make it opt-in.

Task-number: QTBUG-61384
Change-Id: I107f06a881a34fa711b386265d8dc2edfb246624
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>