xdg shell: add wlr_xdg_surface_popup_surface_at()

This function will allow compositors to implement input handling in a
way consistent with rendering more easily.

Calling wlr_xdg_surface_surface_at() and checking if the result is a
wlr_xdg_popup is flawed as there may be subsurfaces in the popup tree.

include/wlr/types/wlr_xdg_shell.h

@@ -369,6 +369,15 @@ struct wlr_surface *wlr_xdg_surface_surface_at(
 		struct wlr_xdg_surface *surface, double sx, double sy,
 		double *sub_x, double *sub_y);
 
+/**
+ * Find a surface within this xdg-surface's popup tree at the given
+ * surface-local coordinates. Returns the surface and coordinates in the leaf
+ * surface coordinate system or NULL if no surface is found at that location.
+ */
+struct wlr_surface *wlr_xdg_surface_popup_surface_at(
+		struct wlr_xdg_surface *surface, double sx, double sy,
+		double *sub_x, double *sub_y);
+
 bool wlr_surface_is_xdg_surface(struct wlr_surface *surface);
 
 struct wlr_xdg_surface *wlr_xdg_surface_from_wlr_surface(

types/xdg_shell/wlr_xdg_surface.c

@@ -583,6 +583,17 @@ static void xdg_popup_get_position(struct wlr_xdg_popup *popup,
 struct wlr_surface *wlr_xdg_surface_surface_at(
 		struct wlr_xdg_surface *surface, double sx, double sy,
 		double *sub_x, double *sub_y) {
+	struct wlr_surface *sub = wlr_xdg_surface_popup_surface_at(surface, sx, sy,
+			sub_x, sub_y);
+	if (sub != NULL) {
+		return sub;
+	}
+	return wlr_surface_surface_at(surface->surface, sx, sy, sub_x, sub_y);
+}
+
+struct wlr_surface *wlr_xdg_surface_popup_surface_at(
+		struct wlr_xdg_surface *surface, double sx, double sy,
+		double *sub_x, double *sub_y) {
 	struct wlr_xdg_popup *popup_state;
 	wl_list_for_each(popup_state, &surface->popups, link) {
 		struct wlr_xdg_surface *popup = popup_state->base;
@@ -599,7 +610,7 @@ struct wlr_surface *wlr_xdg_surface_surface_at(
 		}
 	}
 
-	return wlr_surface_surface_at(surface->surface, sx, sy, sub_x, sub_y);
+	return NULL;
 }
 
 struct xdg_surface_iterator_data {