window-actor: Kill off needs_pixmap

It's mostly equivalent to the case where we've already detached
the pixmap, *except* for the x11_size_changed case. We can simply
detach the pixmap at the time the window changes size, though.

https://bugzilla.gnome.org/show_bug.cgi?id=720631
window-actor: Split into two subclasses of MetaSurfaceActor

The rendering logic before was somewhat complex. We had three independent
cases to take into account when doing rendering:

  * X11 compositor. In this case, we're a traditional X11 compositor,
    not a Wayland compositor. We use XCompositeNameWindowPixmap to get
    the backing pixmap for the window, and deal with the COMPOSITE
    extension messiness.

    In this case, meta_is_wayland_compositor() is FALSE.

  * Wayland clients. In this case, we're a Wayland compositor managing
    Wayland surfaces. The rendering for this is fairly straightforward,
    as Cogl handles most of the complexity with EGL and SHM buffers...
    Wayland clients give us the input and opaque regions through
    wl_surface.

    In this case, meta_is_wayland_compositor() is TRUE and
    priv->window->client_type == META_WINDOW_CLIENT_TYPE_WAYLAND.

  * XWayland clients. In this case, we're a Wayland compositor, like
    above, and XWayland hands us Wayland surfaces. XWayland handles
    the COMPOSITE extension messiness for us, and hands us a buffer
    like any other Wayland client. We have to fetch the input and
    opaque regions from the X11 window ourselves.

    In this case, meta_is_wayland_compositor() is TRUE and
    priv->window->client_type == META_WINDOW_CLIENT_TYPE_X11.

We now split the rendering logic into two subclasses, which are:

  * MetaSurfaceActorX11, which handles the X11 compositor case, in that
    it uses XCompositeNameWindowPixmap to get the backing pixmap, and
    deal with all the COMPOSITE extension messiness.

  * MetaSurfaceActorWayland, which handles the Wayland compositor case
    for both native Wayland clients and XWayland clients. XWayland handles
    COMPOSITE for us, and handles pushing a surface over through the
    xf86-video-wayland DDX.

Frame sync is still in MetaWindowActor, as it needs to work for both the
X11 compositor and XWayland client cases. When Wayland's video display
protocol lands, this will need to be significantly overhauled, as it would
have to work for any wl_surface, including subsurfaces, so we would need
surface-level discretion.

https://bugzilla.gnome.org/show_bug.cgi?id=720631