shell: Explicitly use solid weston_buffers

Rather than punching through to set the surface as a solid colour,
attach an actual weston_buffer to it instead.

This becomes the first user of attaching non-client-generated buffers
to a weston_surface. As a result, it is necessary to introduce a
function which will allow compositors and shells to attach a buffer to a
surface. weston_surface_attach_solid() is therefore introduced as a
special-purpose helper which will attach a solid buffer to a
weston_surface.

It is not intended as a general-purpose mechanism to allow compositors
to attach client-generated buffers to surfaces, as doing so would have
unknown effects on this core part of the compositor itself.

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

include/libweston/libweston.h

@@ -1776,6 +1776,11 @@ struct weston_buffer_reference *
 weston_buffer_create_solid_rgba(struct weston_compositor *compositor,
 				float r, float g, float b, float a);
 
+void
+weston_surface_attach_solid(struct weston_surface *surface,
+			    struct weston_buffer_reference *buffer_ref,
+			    int w, int h);
+
 void
 weston_buffer_destroy_solid(struct weston_buffer_reference *buffer_ref);