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.
