libweston: Store shm buffer stride in weston_buffer

After c08a6ff8 moved attach to the render loop, we have a bad situation
when clients delete an attached shm buffer. We try to query the stride
at attach time, but the shm_buffer has been destroyed, and we crash.

Instead of carefully fixing that, I've instead stored the stride at
buffer creation time (as we already do with buffer width and height).
This lets attach succeed in the gl-renderer, keeping the old texture data
available for any upcoming rendering.

Fixes: #927

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>
Tested-by: Connor Abbott <cwabbott0@gmail.com>