wayland-surface: Fix typo
wayland-surface: Don't clean up the buffer_destroy_listener twice

double_buffered_state_destroy will do this for us.
wayland-surface: Repick after cleaning up everything else

Just a code cleanup to keep all the freeing code together.
wayland-surface: Use the same commit() when commiting subsurface

Refactor our commit() implementation out of wl_surface_commit(),
and pass the double-buffered state around to all our implementation
methods. This allows us to drop a few lines destroying and
reinitializing list of frame callbacks. This is a big cleanup for
the next commit, though.
wayland-surface: Attach the buffer to a surface, even if it doesn't have a role

Currently, set_cursor isn't properly working. Often, the requests look
like this:

  cursor_surface = wl_compositor.create_surface()
  cursor_buffer = create_cursor_buffer()
  cursor_surface.attach(cursor_buffer, 0, 0)
  cursor_surface.commit()
  wl_pointer.set_cursor(cursor_surface, 7, 14)

But since the surface doesn't "have a role" when the commit comes in,
we ignore it, and don't do anything with the pending buffer. When
set_cursor is called, however, we don't immediately update anything
since it doesn't have a buffer.

This effectively means that set_cursor has unpredictable side effects.
Weston's toy toolkit reuses the same surface for every buffer, so it
only fails the first time. In clients that use a new surface for every
cursor sprite, the cursor is effectively invisible.

To solve this, change the code to always set the buffer for a surface,
even if it doesn't have any real role.
wayland-seat: Fix pointer issues

Moving the mouse over weston-terminal, we can see several issues:

 * it often updates late, or not at all
 * the attachment of the pointer sprite is wrong

These are because we willy-nilly call seat_update_sprite all over the
place, and often in wrong areas. Set up a set_pointer_surface helper
method that will do the right thing for us in all cases, and call it
on transitions.
wayland-seat: Rename sprite to cursor_surface
wayland: Simply store a MetaWaylandBuffer

There's no need for the MetaWaylandBufferReference abstraction...