window: Add API to check for subcompositor, use in subsurface example

This way we can fail with a nice error message.

clients/subsurfaces.c

@@ -781,6 +781,12 @@ main(int argc, char *argv[])
 		return -1;
 	}
 
+	if (!display_has_subcompositor(display)) {
+		fprintf(stderr, "compositor does not support "
+			"the subcompositor extension\n");
+		return -1;
+	}
+
 	app = demoapp_create(display);
 
 	display_run(display);

clients/window.c

@@ -5367,6 +5367,17 @@ display_get_display(struct display *display)
 	return display->display;
 }
 
+int
+display_has_subcompositor(struct display *display)
+{
+	if (display->subcompositor)
+		return 1;
+
+	wl_display_roundtrip(display->display);
+
+	return display->subcompositor != NULL;
+}
+
 cairo_device_t *
 display_get_cairo_device(struct display *display)
 {

clients/window.h

@@ -78,6 +78,9 @@ display_get_user_data(struct display *display);
 struct wl_display *
 display_get_display(struct display *display);
 
+int
+display_has_subcompositor(struct display *display);
+
 cairo_device_t *
 display_get_cairo_device(struct display *display);