QtQuick Animator animations + threaded render loop bad performance on Wayland

This is same as QTBUG-72578 which was closed, but the bad performance is still there.

Any QML scene with Animator animations will stutter heavily on Wayland when used with threaded render loop. Switching to basic render loop fixes the performance. It only happens on Wayland platform (under any compositor - tested under weston, sway and QtWaylandCompositor), works perfectly fine with threaded render loop on xcb, eglfs, etc.

Attached is an example that will trigger the issue - simply try to scroll inside the window.

# Choppy animation
QSG_RENDER_LOOP=threaded ./scene6

# Smooth animation
QSG_RENDER_LOOP=basic ./scene6