Use the common infrastructure we have, rather than open-coding again.

In changing to the common weston_curtain infrastructure which was
introduced in a previous commit, there are two small functional
derivations.

After adding the view to a layer, we explicitly call
weston_view_geometry_dirty(). This is believed to have no functional
impact, as weston_views have their geometry-dirty flag set when they are
created.

weston_surface_damage() is also called to ensure that the surface is
scheduled for a repaint. As there is currently no good common mechanic
in the common code to ensure that output repaint will occur, due to the
damage propagating outwards from the weston_surface, we are forced to
call this to predictably ensure that the output will be repainted after
we have made this change to the scene graph which should result in the
user observing the new content being repainted.

It is possible that these changes are not strictly required in order for
the correct behaviour to occur. However, it is felt that explicitly
adding these calls is perhaps the least fragile way to ensure that the
behaviour continues to be correct and breakage is not observed,
especially with both view mapping and surface damage identified as areas
for future work which could be beneficial to Weston. If it is felt that
these calls can be removed, then this is certainly possible at a later
stage.

Lastly, there are many users within desktop-shell of the common pattern
of creating a weston_curtain and inserting it into the scene graph to be
painted. These users have been an area of both theoretical concern and
real-life observed fragility and breakage recently. By making each user
follow a common and predictable pattern of usage, each user is no longer
its own special case. This should make it easier to make the
desktop-shell codebase more robust, not only simplifying the codebase,
but improving observed behaviour.

In doing this, it is hoped that future structural and interface changes
become easier to make, allowing us to improve some of the more difficult
corners of the libweston API.

Signed-off-by: Daniel Stone <daniels@collabora.com>