effects/screenshot: Port to QPromise

QFutureInterface is technically internal Qt API and the screenshot uses
it solely because we knew that there won't be any future Qt 5 releases
and it won't be removed in 5.15 patch releases.

In Qt 6, we have proper public API. Unlike QFutureInterface, QPromise is
not copyable, therefore this change also ports some parts of the screenshot
effect from QVector, which uses copy-on-write, to std::vector.