Give the length tuning tool an inital end point that isn't the start point

Using the starting point as the end point means that the tool
initializes with 0 length traces and so nothing is visible.

Fixes https://gitlab.com/kicad/code/kicad/issues/1912

pcbnew/router/length_tuner_tool.cpp

@@ -180,7 +180,7 @@ void LENGTH_TUNER_TOOL::performTuning()
 
     placer->UpdateSettings( m_savedMeanderSettings );
 
-    VECTOR2I end( m_startSnapPoint );
+    VECTOR2I end = getViewControls()->GetMousePosition();
 
     // Create an instance of PNS_TUNE_STATUS_POPUP.
     PNS_TUNE_STATUS_POPUP statusPopup( frame() );