kwin: move/resize events shall not force a repaint of all layers

This patch adds a new function Toplevel::addLayerRepaint, that in contrast
to addWorkspaceRepaint does not invalidate every blur texture cache that
overlaps with that region. As the name suggests it rather invalidates the
to the window associated layer at that position. This is especially useful
in the case of move/resize events in combination with oxygen-transparent,
where the altered window is almost always the topmost window and the blur
texture cache of the windows underneath are unchanged.

For the case of fully opaque windows the behaviour of addLayerRepaint
and addWorkspaceRepaint should be same.

REVIEW: 103906