plugins/screenshot: Cache screenshot attributes

When a QPromise reports results, it's not necessary that the
QFutureWatcher is going to report it immediately. That can happen at
some point in the future, which is okay according to the QFuture api
contract.

Due to that, we cannot assume that the stored Output and EffectWindow
objects pointers are valid when the QFutureWatcher::finished is emitted.


(cherry picked from commit 69f344a4)