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.

src/render.c

@@ -232,7 +232,7 @@ static void render_root(Con *con, Con *fullscreen) {
             continue;
         }
         Con *workspace = TAILQ_FIRST(&(content->focus_head));
-        Con *fullscreen = con_get_fullscreen_con(workspace, CF_OUTPUT);
+        Con *fullscreen = con_get_fullscreen_covering_ws(workspace);
         Con *child;
         TAILQ_FOREACH(child, &(workspace->floating_head), floating_windows) {
             if (fullscreen != NULL) {