subsurface: rename wlr_subsurface_state to wlr_subsurface_parent_state

Add a comment to explain the difference.

include/wlr/types/wlr_surface.h

@@ -163,7 +163,12 @@ struct wlr_surface {
 	} previous;
 };
 
-struct wlr_subsurface_state {
+/**
+ * The sub-surface state describing the sub-surface's relationship with its
+ * parent. Contrary to other states, this one is not applied on surface commit.
+ * Instead, it's applied on parent surface commit.
+ */
+struct wlr_subsurface_parent_state {
 	int32_t x, y;
 };
 
@@ -172,7 +177,7 @@ struct wlr_subsurface {
 	struct wlr_surface *surface;
 	struct wlr_surface *parent;
 
-	struct wlr_subsurface_state current, pending;
+	struct wlr_subsurface_parent_state current, pending;
 
 	uint32_t cached_seq;
 	bool has_cache;

types/wlr_surface.c

@@ -1350,7 +1350,7 @@ static void surface_for_each_surface(struct wlr_surface *surface, int x, int y,
 		wlr_surface_iterator_func_t iterator, void *user_data) {
 	struct wlr_subsurface *subsurface;
 	wl_list_for_each(subsurface, &surface->subsurfaces_below, parent_link) {
-		struct wlr_subsurface_state *state = &subsurface->current;
+		struct wlr_subsurface_parent_state *state = &subsurface->current;
 		int sx = state->x;
 		int sy = state->y;
 
@@ -1361,7 +1361,7 @@ static void surface_for_each_surface(struct wlr_surface *surface, int x, int y,
 	iterator(surface, x, y, user_data);
 
 	wl_list_for_each(subsurface, &surface->subsurfaces_above, parent_link) {
-		struct wlr_subsurface_state *state = &subsurface->current;
+		struct wlr_subsurface_parent_state *state = &subsurface->current;
 		int sx = state->x;
 		int sy = state->y;