Don't dispatch drag start and move together

Drag start is responsible for switching into drag mode, classing
stuff as `active` and kicking off a bunch of other things.

If the drag move happens immediately after this, and includes the
target from the initial active drag, it can cause weird snapping
from the dragnode to its own parent way.  (Happened if the user did
a very fast drag from the node along the parent way just next to it)

modules/behavior/drag.js

@@ -110,19 +110,19 @@ export function behaviorDrag() {
             if (dx === 0 && dy === 0)
                 return;
 
-            if (!started) {
-                started = true;
-                _event({ type: 'start' });
-            }
-
             startOrigin = p;
             d3_eventCancel();
 
-            _event({
-                type: 'move',
-                point: [p[0] + offset[0],  p[1] + offset[1]],
-                delta: [dx, dy]
-            });
+            if (!started) {
+                started = true;
+                _event({ type: 'start' });
+            } else {
+                _event({
+                    type: 'move',
+                    point: [p[0] + offset[0],  p[1] + offset[1]],
+                    delta: [dx, dy]
+                });
+            }
         }