Handle new QWindow paint events in Qt Quick render loops

QTBUG-82676 will likely introduce a new type of event for QWindow, the Paint event. This can be sent even when being unexposed (on some platforms), and is replacing expose on resize. Later on the paint event is expected to replace UpdateRequest (that is sent in response requestUpdate) as well.

Due to compatibility measures Qt Quick will likely work normally without any changes, possibly with a visual regression due to missing the first frame on Apple platforms. We should however investigate how the new paint event can be handled both in the 'basic' and 'threaded' loops, in order to have a fully correct rendering logic that plays nice with all platforms.