When we first see a buffer attached, we create a weston_buffer for it.
The weston_buffer doesn't contain any useful information in and of
itself; that's left to renderers to populate later.
Switch this to doing it in the core at the first opportunity, at least
for SHM and dmabuf buffers; EGL buffers will follow in the next commit.
Signed-off-by: Daniel Stone <daniels@collabora.com>