Avoid painting unchanged scene layers

Currently when we move the mouse the one render loop triggers a repaint.
When the cursor layer needs a new update we end up in the compositor
repainting the main content.

Even though painting should mostly no-op it still goes through all
existing items and effects to collect damage, still potentially making
the GL context current which could stall. A waste when we know we
haven't got anything to do. It's enough to cause noticable mouse lag on
some hardware.

Co-authored-by: Vlad Zahorodnii <vlad.zahorodnii@kde.org>