Introduce con_get_fullscreen_covering_ws

This commit will also fix the following bugs:
1. click.c: Users could drag global fullscreen floating containers.
2. render.c: Floating containers would get rendered with a global fullscreen container in another
workspace.

include/con.h

@@ -120,6 +120,14 @@ Con *con_parent_with_orientation(Con *con, orientation_t orientation);
  */
 Con *con_get_fullscreen_con(Con *con, fullscreen_mode_t fullscreen_mode);
 
+/**
+ * Returns the fullscreen node that covers the given workspace if it exists.
+ * This is either a CF_GLOBAL fullscreen container anywhere or a CF_OUTPUT
+ * fullscreen container in the workspace.
+ *
+ */
+Con *con_get_fullscreen_covering_ws(Con *ws);
+
 /**
  * Returns true if the container is internal, such as __i3_scratch
  *