compositor: Split dnd setup up into weston_seat_start_drag()

This makes the drag-and-drop code available to in-weston data sources,
similar to how we can set a selection data source internally.  The
wl_data_device.start_drag entry point now calls this function after
validating protocol arguments.

src/compositor.h

@@ -392,6 +392,11 @@ wl_data_device_manager_init(struct wl_display *display);
 void
 weston_seat_set_selection(struct weston_seat *seat,
 			  struct weston_data_source *source, uint32_t serial);
+int
+weston_seat_start_drag(struct weston_seat *seat,
+		       struct weston_data_source *source,
+		       struct weston_surface *icon,
+		       struct wl_client *client);
 
 struct weston_xkb_info {
 	struct xkb_keymap *keymap;

src/data-device.c

@@ -358,6 +358,43 @@ handle_drag_icon_destroy(struct wl_listener *listener, void *data)
 	drag->icon = NULL;
 }
 
+WL_EXPORT int
+weston_seat_start_drag(struct weston_seat *seat,
+		       struct weston_data_source *source,
+		       struct weston_surface *icon,
+		       struct wl_client *client)
+{
+	struct weston_drag *drag;
+
+	drag = zalloc(sizeof *drag);
+	if (drag == NULL)
+		return -1;
+
+	drag->grab.interface = &drag_grab_interface;
+	drag->client = client;
+	drag->data_source = source;
+	drag->icon = icon;
+
+	if (source) {
+		drag->data_source_listener.notify = destroy_data_device_source;
+		wl_signal_add(&source->destroy_signal,
+			      &drag->data_source_listener);
+	}
+
+	if (icon) {
+		drag->icon_destroy_listener.notify = handle_drag_icon_destroy;
+		wl_signal_add(&icon->destroy_signal,
+			      &drag->icon_destroy_listener);
+
+		icon->configure = drag_surface_configure;
+		icon->configure_private = drag;
+	}
+
+	weston_pointer_set_focus(seat->pointer, NULL,
+				 wl_fixed_from_int(0), wl_fixed_from_int(0));
+	weston_pointer_start_grab(seat->pointer, &drag->grab);
+}
+
 static void
 data_device_start_drag(struct wl_client *client, struct wl_resource *resource,
 		       struct wl_resource *source_resource,
@@ -365,7 +402,7 @@ data_device_start_drag(struct wl_client *client, struct wl_resource *resource,
 		       struct wl_resource *icon_resource, uint32_t serial)
 {
 	struct weston_seat *seat = wl_resource_get_user_data(resource);
-	struct weston_drag *drag;
+	struct weston_data_source *source;
 	struct weston_surface *icon = NULL;
 
 	if (seat->pointer->button_count == 0 ||
@@ -375,6 +412,8 @@ data_device_start_drag(struct wl_client *client, struct wl_resource *resource,
 
 	/* FIXME: Check that the data source type array isn't empty. */
 
+	if (source_resource)
+		source = wl_resource_get_user_data(source_resource);
 	if (icon_resource)
 		icon = wl_resource_get_user_data(icon_resource);
 	if (icon && icon->configure) {
@@ -384,35 +423,8 @@ data_device_start_drag(struct wl_client *client, struct wl_resource *resource,
 		return;
 	}
 
-	drag = zalloc(sizeof *drag);
-	if (drag == NULL) {
+	if (weston_seat_start_drag(seat, source, icon, client) < 0)
 		wl_resource_post_no_memory(resource);
-		return;
-	}
-
-	drag->grab.interface = &drag_grab_interface;
-	drag->client = client;
-
-	if (source_resource) {
-		drag->data_source = wl_resource_get_user_data(source_resource);
-		drag->data_source_listener.notify = destroy_data_device_source;
-		wl_resource_add_destroy_listener(source_resource,
-						 &drag->data_source_listener);
-	}
-
-	if (icon) {
-		drag->icon = icon;
-		drag->icon_destroy_listener.notify = handle_drag_icon_destroy;
-		wl_signal_add(&icon->destroy_signal,
-			      &drag->icon_destroy_listener);
-
-		icon->configure = drag_surface_configure;
-		icon->configure_private = drag;
-	}
-
-	weston_pointer_set_focus(seat->pointer, NULL,
-				 wl_fixed_from_int(0), wl_fixed_from_int(0));
-	weston_pointer_start_grab(seat->pointer, &drag->grab);
 }
 
 static void