nested: Remove the surface from the surface list when destroyed

Otherwise if the surface is destroyed then it will crash when it later
tries to render all of the surfaces. You can replicate this by doing
killall weston-nested-client while the example is running.
nested: Add the buffer reference semantics from Weston

This copies the buffer reference busy count implementation from Weston
to the nested compositor example and adds an internal nested_buffer
struct that we could eventually use to attach data. This will be
useful to adapt the example to use subsurfaces so that we can attach
our compositor-side buffer to the resource.
nested: Add double-buffered state semantics to the nested example

The buffer and frame callback state on the surfaces in the nested
compositor example are now double-buffered so that they only take
effect when the commit request is received. This doesn't really make
much difference for the current state that the example has but it will
be useful when more state is added in later patches.
nested: Add damage tracking to the nested compositor example

The nested compositor example now responds to damage requests and
tracks them in the pending buffer state. This isn't currently used for
anything and it is immediately discarded when the surface is commited
but it will be used later when the example is converted to use
subsurfaces.
nested: Move the frame callback list to the surface

Previously the frame callback list was tracked as part of the global
compositor state. This patch moves the list to be part of the surface
state like it is in Weston. The frame callback now iterates the list
of surfaces to flush all of the callbacks. This change will be useful
when the example is converted to use subsurfaces so that it can have a
separate frame callback for the subsurface and flush the list for an
individual client surface rather than flushing globally.
nested: Add a ‘renderer’ mechanism with a vtable

Eventually the nested compositor example will want to be able to cope
with either rendering as it does now with a blit to an intermediate
surface or by attaching the client buffers directly to a subsurface
without copying. This patch moves the code that is specific to the
blitting mechanism into a separate set of functions with a vtable to
make it easier to add the second way of rendering in a later patch.
clients: Add a widget_get_wl_subsurface

Adds a simple accessor for the wl_subsurface for widgets created with
window_add_subsurface.
nested: Add a renderer using subsurfaces

Adds a second renderer implementation to the nested compositor example
that creates a subsurface for each of the client's surfaces. The
client buffers are directly attached to the subsurface using the
EGL_WL_create_wayland_buffer_from_image extension instead of blitting
them in the redraw_handler.

The new renderer is always used if the parent compositor supports the
wl_subcompositor protocol and the EGL extension is available.
Otherwise it will fall back to the blit renderer.
nested: Add an option to disable subsurfaces

This adds a -b option to force the nested compositor example to use
the old blit renderer even if the appropriate extensions are
available.