i3bar: suspend the child when bars are fully obscured

i3bar/include/outputs.h

@@ -40,6 +40,7 @@ struct i3_output {
     char* name;   /* Name of the output */
     bool active;  /* If the output is active */
     bool primary; /* If it is the primary output */
+    bool visible; /* If the bar is visible on this output */
     int ws;       /* The number of the currently visible ws */
     rect rect;    /* The rect (relative to the root-win) */