Fix the map jumping at the end of some transitions.

Sometimes, when zooming, the map would jump at the end of the transition.  This was caused by the zoom level reaching its final value, then a last animation frame firing, and asking to recenter the map when the center was already correct.

src/map.js

@@ -1109,7 +1109,7 @@ var map = function (arg) {
     }
 
     var defaultOpts = {
-      center: m_this.center(undefined, null),
+      center: undefined,
       zoom: m_this.zoom(),
       rotation: m_this.rotation(),
       duration: 1000,
@@ -1155,8 +1155,8 @@ var map = function (arg) {
       opts.zCoord ? zoom2z(m_transition.start.zoom) : m_transition.start.zoom,
       m_transition.start.rotation
     ], [
-      m_transition.end.center.x,
-      m_transition.end.center.y,
+      m_transition.end.center ? m_transition.end.center.x : m_transition.start.center.x,
+      m_transition.end.center ? m_transition.end.center.y : m_transition.start.center.y,
       opts.zCoord ? zoom2z(m_transition.end.zoom) : m_transition.end.zoom,
       m_transition.end.rotation
     ]);
@@ -1192,8 +1192,10 @@ var map = function (arg) {
       m_transition.time = time - m_transition.start.time;
       if (time >= m_transition.end.time || next) {
         if (!next) {
-          var needZoom = m_zoom !== fix_zoom(m_transition.end.zoom);
-          m_this.center(m_transition.end.center, null, needZoom, needZoom);
+          if (m_transition.end.center) {
+            var needZoom = m_zoom !== fix_zoom(m_transition.end.zoom);
+            m_this.center(m_transition.end.center, null, needZoom, needZoom);
+          }
           m_this.zoom(m_transition.end.zoom, m_transition.zoomOrigin);
           m_this.rotation(fix_rotation(m_transition.end.rotation));
         }