weston_buffer: Prepare for buffer to outlive resource

We currently allow a weston_buffer to outlive the underlying wl_buffer
iff the renderer/backend has cached it. Currently the 'is this buffer
valid?' test relies on looking for the validity of the weston_buffer
itself; shift these tests to looking at the validity of the underlying
resource.

Signed-off-by: Daniel Stone <daniels@collabora.com>

libweston/compositor.c

@@ -1983,7 +1983,11 @@ weston_view_is_opaque(struct weston_view *ev, pixman_region32_t *region)
 WL_EXPORT bool
 weston_view_has_valid_buffer(struct weston_view *ev)
 {
-	return ev->surface->buffer_ref.buffer != NULL;
+	if (!ev->surface->buffer_ref.buffer)
+		return false;
+	if (!ev->surface->buffer_ref.buffer->resource)
+		return false;
+	return true;
 }
 
 /** Check if the view matches the entire output

libweston/data-device.c

@@ -420,6 +420,7 @@ drag_surface_configure(struct weston_drag *drag,
 	assert((pointer != NULL && touch == NULL) ||
 			(pointer == NULL && touch != NULL));
 
+	/* XXX: Why are we checking for a valid buffer here too ... ? */
 	if (!weston_surface_is_mapped(es) && es->buffer_ref.buffer) {
 		if (pointer && pointer->sprite &&
 			weston_view_is_mapped(pointer->sprite))