Use microseconds instead of miliseconds to avoid discontinuities (#39)

Author: ozziek

lib/src/page_list_viewport.dart

@@ -505,7 +505,7 @@ class PageListViewportController extends OrientationController {
 
     // Update velocity tracking.
     if (_velocityStopwatch.elapsedMilliseconds > 0) {
-      _velocity = (newOrigin - _previousOrigin) / (_velocityStopwatch.elapsedMilliseconds / 1000);
+      _velocity = (newOrigin - _previousOrigin) / (_velocityStopwatch.elapsedMicroseconds / 1000000);
       _velocityStopwatch.reset();
       _velocityResetTimer?.cancel();
 

lib/src/page_list_viewport_gestures.dart

@@ -274,7 +274,7 @@ class _PageListViewportGesturesState extends State<PageListViewportGestures> wit
       return;
     }
 
-    final secondsFraction = elapsedTime.inMilliseconds / 1000;
+    final secondsFraction = elapsedTime.inMicroseconds / 1000000;
     final currentVelocity = _frictionSimulation!.dx(secondsFraction);
     final originBeforeDelta = widget.controller.origin;
     final newOrigin = _frictionSimulation!.x(secondsFraction);