We tried to synchronize layer shell popups with the parent layer shell
on commits, but this is subtly wrong because we would only update
the position for one layer shell that was committed, but not any other
layer that might be affected. By moving handling to the scene descriptor
we can iterate all popups and ensure they are synchronized.
This allows for layer shell surfaces to receive focus while the surface is explicitly focused, i.e allowing
text fields to receive keyboard input just like a regular surface.
This commit makes sure the extents are kept up-to-date, fixes not
damaging the surface if its layer shell-specific state didn't change,
and adds a check if the layer shell-specific state didn't change but the
surface got mapped/unmapped, which could affect keyboard focus.
When a layer surface shrinks we need to damage the area it previously
occupied, but we don't know the location of all its subsurfaces in the
previous state, so instead damage a rectangle that encloses the entire
previous extent.
