zoom: Call weston_output_activate_zoom() appropriately

No longer call weston_output_update_zoom() when trying to zoom out
on an unzoomed output.

Add an assert() to make sure update_zoom is never called without an
active zoom.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>

src/zoom.c

@@ -25,6 +25,7 @@
 
 #include "config.h"
 
+#include <assert.h>
 #include <stdlib.h>
 #include <stdbool.h>
 
@@ -135,6 +136,8 @@ weston_output_update_zoom(struct weston_output *output)
 {
 	struct weston_seat *seat = weston_zoom_pick_seat(output->compositor);
 
+	assert(output->zoom.active);
+
 	output->zoom.current.x = seat->pointer->x;
 	output->zoom.current.y = seat->pointer->y;