Pcbnew: Prevent selections jumping when dragged after flip or rotate

CHANGED: If you flip or rotate an item or selection of items, move
the mouse, press 'M' and start moving the mouse, the selection jumps
to where the mouse is.  This MR fixes that so it doesn't happen.

This bug was reported against the 5.1.x branch but also affects the
5.99 branch.  This MR is for the 5.99 branch.

Fixes https://gitlab.com/kicad/code/kicad/issues/4069

pcbnew/tools/edit_tool.cpp

@@ -1002,6 +1002,10 @@ int EDIT_TOOL::Rotate( const TOOL_EVENT& aEvent )
     if( m_dragging )
         m_toolMgr->RunAction( PCB_ACTIONS::updateLocalRatsnest, false );
 
+    // Clear reference so any mouse dragging that occurs doesn't make the selection jump
+    // to this now invalid reference
+    selection.ClearReferencePoint();
+
     return 0;
 }
 
@@ -1224,6 +1228,10 @@ int EDIT_TOOL::Flip( const TOOL_EVENT& aEvent )
     if( m_dragging )
         m_toolMgr->RunAction( PCB_ACTIONS::updateLocalRatsnest, false );
 
+    // Clear reference so any mouse dragging that occurs doesn't make the selection jump
+    // to this now invalid reference
+    selection.ClearReferencePoint();
+
     return 0;
 }