tests/wayland-test-clients: Use stride of the first plane

The assumption is that all planes are always contiguous, and we don't
have any multi-plane formats where the first plane is subsampled.

The stride of the entire buffer is then just the stride of the first
plane and the stride of the other planes is derived from that.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3371>

src/tests/wayland-test-clients/wayland-test-client-utils.c

@@ -973,7 +973,7 @@ wayland_buffer_shm_allocate (WaylandBuffer *buffer,
   pool = wl_shm_create_pool (display->shm, fd, shm->size);
   priv->buffer = wl_shm_pool_create_buffer (pool, 0,
                                             priv->width, priv->height,
-                                            shm->size / priv->height,
+                                            shm->stride[0],
                                             shm_format);
   wl_shm_pool_destroy (pool);