Find and hold mask texture of render passes

Even if we don't yet implement handling it, we need to hold it to not
assert later on it missing from held resources.

Task-number: QTBUG-67414
Change-Id: Ic14efba383ba7f1809238768b386b2254fad7121
Reviewed-by: Jüri Valdmann <juri.valdmann@qt.io>
Reviewed-by: Viktor Engelmann <viktor.engelmann@qt.io>