Add a weston_buffer structure to replace wl_buffer

This commit adds a weston_buffer structure to replace wl_buffer.  This way
we can hold onto buffers by just their resource.  In order to do this, the
every renderer.attach function has to fill in the weston_buffer.width and
weston_buffer.height fields.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
diff --git a/src/noop-renderer.c b/src/noop-renderer.c
index c0765f2..91659f5 100644
--- a/src/noop-renderer.c
+++ b/src/noop-renderer.c
@@ -47,7 +47,7 @@
 }
 
 static void
-noop_renderer_attach(struct weston_surface *es, struct wl_buffer *buffer)
+noop_renderer_attach(struct weston_surface *es, struct weston_buffer *buffer)
 {
 }