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 (closed)

Signed-off-by: Derek Foreman derek.foreman@collabora.com