evdev: use EV_SYN/SYN_REPORT for dispatching motion events

We can't dispatch a motion event for EV_REL (because we don't
have yet the other half of the event), but we can't also queue
them at the end of processing (because we may lose some history
or have button/keys intermixed).
Instead, we use EV_SYN, which means "one logical event was
completed", and let the winsys-independent code do the actual
motion compression.

https://bugzilla.gnome.org/show_bug.cgi?id=706494

clutter/evdev/clutter-device-manager-evdev.c

@@ -129,6 +129,8 @@ struct _ClutterEventSource
   ClutterInputDeviceEvdev *device;    /* back pointer to the slave evdev device */
   GPollFD event_poll_fd;              /* file descriptor of the /dev node */
   struct libevdev *dev;
+
+  int dx, dy;
 };
 
 static gboolean
@@ -447,9 +449,7 @@ notify_button (ClutterEventSource *source,
 
 static void
 dispatch_one_event (ClutterEventSource *source,
-		    struct input_event *e,
+		    struct input_event *e)
-		    int                *dx,
-		    int                *dy)
 {
   guint32 _time;
 
@@ -467,12 +467,26 @@ dispatch_one_event (ClutterEventSource *source,
 	    notify_button (source, _time, e->code, e->value);
 	}
       else
-	/* We don't know about this code, ignore */;
+	{
+	  /* We don't know about this code, ignore */
+	}
       break;
 
     case EV_SYN:
+      if (e->code == SYN_REPORT)
+	{
+	  /* Flush accumulated motion deltas */
+	  if (source->dx != 0 || source->dy != 0)
+	    {
+	      notify_relative_motion (source, _time, source->dx, source->dy);
+	      source->dx = 0;
+	      source->dy = 0;
+	    }
+	}
+      break;
+
     case EV_MSC:
-      /* Nothing to do here (actually, EV_SYN is handled by libevdev, we shouldn't see it) */
+      /* Nothing to do here */
       break;
 
     case EV_REL:
@@ -480,10 +494,10 @@ dispatch_one_event (ClutterEventSource *source,
       switch (e->code)
 	{
 	case REL_X:
-	  *dx += e->value;
+	  source->dx += e->value;
 	  break;
 	case REL_Y:
-	  *dy += e->value;
+	  source->dy += e->value;
 	  break;
 
 	case REL_WHEEL:
@@ -507,14 +521,13 @@ sync_source (ClutterEventSource *source)
 {
   struct input_event ev;
   int err;
-  int dx = 0, dy = 0;
   const gchar *device_path;
 
   /* We read a SYN_DROPPED, ignore it and sync the device */
   err = libevdev_next_event (source->dev, LIBEVDEV_READ_SYNC, &ev);
   while (err == 1)
     {
-      dispatch_one_event (source, &ev, &dx, &dy);
+      dispatch_one_event (source, &ev);
       err = libevdev_next_event (source->dev, LIBEVDEV_READ_SYNC, &ev);
     }
 
@@ -524,12 +537,6 @@ sync_source (ClutterEventSource *source)
 
       CLUTTER_NOTE (EVENT, "Could not sync device (%s).", device_path);
     }
-
-  if (dx != 0 || dy != 0)
-    {
-      guint32 _time = ev.time.tv_sec * 1000 + ev.time.tv_usec / 1000;
-      notify_relative_motion (source, _time, dx, dy);
-    }
 }
 
 static void
@@ -565,7 +572,7 @@ clutter_event_dispatch (GSource     *g_source,
   struct input_event ev;
   ClutterEvent *event;
   ClutterStage *stage;
-  int err, dx = 0, dy = 0;
+  int err;
 
   _clutter_threads_acquire_lock ();
 
@@ -582,7 +589,7 @@ clutter_event_dispatch (GSource     *g_source,
       if (err == 1)
 	sync_source (source);
       else if (err == 0)
-	dispatch_one_event (source, &ev, &dx, &dy);
+	dispatch_one_event (source, &ev);
       else
 	{
 	  fail_source (source, -err);
@@ -592,12 +599,6 @@ clutter_event_dispatch (GSource     *g_source,
       err = libevdev_next_event (source->dev, LIBEVDEV_READ_NORMAL, &ev);
     }
 
-  if (dx != 0 || dy != 0)
-    {
-      guint32 _time = ev.time.tv_sec * 1000 + ev.time.tv_usec / 1000;
-      notify_relative_motion (source, _time, dx, dy);
-    }
-
  queue_event:
   /* Drop events if we don't have any stage to forward them to */
   if (stage == NULL)