ssd: attach LAB_NODE_SSD_ROOT to ssd->tree

This doesn't change any behaviors.

Attaching LAB_NODE_NONE node-descriptor to ssd->tree looks strange, this
patch uses new LAB_NODE_SSD_ROOT instead. The node-descriptor attached to
ssd->tree is needed for get_cursor_context() to detect cursor hovering on
borders/extents.

I also updated get_cursor_context() to make my intent clearer.

include/common/node-type.h

@@ -59,6 +59,12 @@ enum lab_node_type {
 	LAB_NODE_LAYER_POPUP,
 	LAB_NODE_SESSION_LOCK_SURFACE,
 	LAB_NODE_IME_POPUP,
+
+	/*
+	 * translated to LAB_CORNER_* or LAB_BORDER* by
+	 * ssd_get_resizing_type()
+	 */
+	LAB_NODE_SSD_ROOT,
 };
 
 enum lab_node_type node_type_parse(const char *context);