compositor: Added layout_save/layout_restore

compositor: Added layout_save/layout_restore

In order to send correct dimensions after a hot-plug event, we need to
be able to save the area we had before.

Note that implies that the connectors do not change names in between
hot-plugs, which normally doesn't happen, but it might if udev rules are
executed/invoked. We restore based on output name it had previously.

Bug-AGL: SPEC-4705
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: Ice3a127edfc9d356830ba8f0d0a20bb86284d7de
(cherry picked from commit 28ec0cff16d62260fb1a5900f57353f48446e199)