libweston: prevent double weston_output_enable()

Enabling an already enabled output is an error, at least with the
current implementation.

However, disabling an output that has not been enabled is ok.

Cope with the first and document the second.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Armin Krezović <krezovic.armin@gmail.com>

libweston/compositor.c

@@ -4712,7 +4712,8 @@ weston_compositor_add_pending_output(struct weston_output *output,
 
 /** Constructs a weston_output object that can be used by the compositor.
  *
- * \param output The weston_output object that needs to be enabled.
+ * \param output The weston_output object that needs to be enabled. Must not
+ * be enabled already.
  *
  * Output coordinates are calculated and each new output is by default
  * assigned to the right of previous one.
@@ -4749,6 +4750,12 @@ weston_output_enable(struct weston_output *output)
 	struct weston_output *iterator;
 	int x = 0, y = 0;
 
+	if (output->enabled) {
+		weston_log("Error: attempt to enable an enabled output '%s'\n",
+			   output->name);
+		return -1;
+	}
+
 	iterator = container_of(c->output_list.prev,
 				struct weston_output, link);
 
@@ -4829,6 +4836,10 @@ weston_output_enable(struct weston_output *output)
  *
  * See weston_output_init() for more information on the
  * state output is returned to.
+ *
+ * If the output has never been enabled yet, this function can still be
+ * called to ensure that the output is actually turned off rather than left
+ * in the state it was discovered in.
  */
 WL_EXPORT void
 weston_output_disable(struct weston_output *output)