swaybar: properly draw blocks with transparent black border

while the draw itself is a no-op, alignment must still be accounted
this requires more signalling about the blocks (border_set; was the
border set?)

include/swaybar/i3bar.h

@@ -19,6 +19,7 @@ struct i3bar_block {
 	// Airblader features
 	uint32_t background;
 	uint32_t border;
+	bool border_set;
 	int border_top;
 	int border_bottom;
 	int border_left;