backend/wayland: Account for shm buffer offset

If we are trying directly scan-out a shm buffer, we create a temporary
buffer pool to instantiate a wl_buffer from, created in accordance to
our buffers size. If the buffer has an offset, it will end up out of
bounds of the created pool.

Extend the temporary pool by the buffer offset to compensate. Matching
the original pool size does matter unless we want to optimize away the
temporary pool to reduce mappings in the parent compositor.

(cherry picked from commit 63fabecee2)

backend/wayland/output.c

@@ -198,7 +198,7 @@ static struct wl_buffer *import_shm(struct wlr_wl_backend *wl,
 		struct wlr_shm_attributes *shm) {
 	enum wl_shm_format wl_shm_format = convert_drm_format_to_wl_shm(shm->format);
 	uint32_t size = shm->stride * shm->height;
-	struct wl_shm_pool *pool = wl_shm_create_pool(wl->shm, shm->fd, size);
+	struct wl_shm_pool *pool = wl_shm_create_pool(wl->shm, shm->fd, shm->offset + size);
 	if (pool == NULL) {
 		return NULL;
 	}