x11: Avoid calls to workspace in OverlayWindow and sync GLX and EGL paths

OverlayWindow had different paths for GLX and EGL when it came to
managing size. On EGL the initial size comes from the overlay resizing
to workspace, but future updates were explicitly handled by the backend.
On GLX the backend tracked changes, but then it was the overlay's job to
resize but using a different method.

This patch also prevents use of workspace after destruction fixing
kwin_x11 --replace.