render_con: Get rid of render_fullscreen argument

Only true for the fullscreen container and doesn't affect any of its
children. Thus, we can get the same result by checking
->fullscreen_mode.

src/tree.c

@@ -453,7 +453,7 @@ void tree_render(void) {
     mark_unmapped(croot);
     croot->mapped = true;
 
-    render_con(croot, false);
+    render_con(croot);
 
     x_push_changes(croot);
     DLOG("-- END RENDERING --\n");