wayland: Fix buffer ref'ing in org_kde_kwin_shadow.commit
In case the current state and the pending state have the same buffer for a particular side or a corner, that buffer can be prematurely released because the buffer in the current state is unreferenced first.
The ref/unref order should be vice versa to ensure that the GraphicsBuffer is not released prematurely.
(cherry picked from commit 1a7c94b6)