Cocoa: Fix backing store performance regression.

Cocoa: Fix backing store performance regression.

Commit 916dfcb8 introduced a regression where QNSView
would hold a copy if the backing store QImage, which
would cause a detach and deep copy if/when the image
was painted on while the copy was held.

Revert back to the previous share-by-pointer scheme.
Make sure the following bugs stays fixed:
    QTBUG-38377
    QTBUG-42206
    QTBUG-44313

Change-Id: Ib8049051d51947113f23b66ee61dc34c22388914
Task-number: QTBUG-46959
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@theqtcompany.com>