ssd: allow ssd to be smaller than minimal size

With 2603dbf labwc refused to update ssd geometry when its geometry is
smaller than minimal size in order to prevent passing negative values in
wlr_scene_rect_set_size(), but it made ssd for small windows like
workrave ugly. So this commit fixes the negative values for each call to
wlr_scene_rect_set_size() individually rather than just early-return in
ssd_update_geometry().

Co-Authored-By: Consolatis <35009135+Consolatis@users.noreply.github.com>

include/ssd-internal.h

@@ -59,6 +59,8 @@ struct ssd {
 		} title;
 	} state;
 
+	struct wlr_scene_rect *view_background;
+
 	/* An invisible area around the view which allows resizing */
 	struct ssd_sub_tree extents;