gtk3: prevent double scroll events

GTK3 smooth scrolling is enabled by wxWidgets but not to the exclusion
of normal scroll up/down events.  wxWidgets maps these both to the same
scroll handler and will fire them both if they are not handled before
being queued.  Testing timestamps allows us to mark and ignore the
dupes.

This is set to GTK3 only for now as it isn't listed as a problem for
other platforms.  But it shouldn't cause issues if it is enabled
elsewhere in the future.

common/view/wx_view_controls.cpp

@@ -52,7 +52,7 @@ static std::unique_ptr<ZOOM_CONTROLLER> GetZoomControllerForPlatform()
 
 WX_VIEW_CONTROLS::WX_VIEW_CONTROLS( VIEW* aView, wxScrolledCanvas* aParentPanel ) :
     VIEW_CONTROLS( aView ), m_state( IDLE ), m_parentPanel( aParentPanel ),
-    m_scrollScale( 1.0, 1.0 ), m_cursorPos( 0, 0 ), m_updateCursor( true )
+    m_scrollScale( 1.0, 1.0 ), m_lastTimestamp( 0 ), m_cursorPos( 0, 0 ), m_updateCursor( true )
 {
     m_parentPanel->Connect( wxEVT_MOTION,
                             wxMouseEventHandler( WX_VIEW_CONTROLS::onMotion ), NULL, this );
@@ -133,6 +133,16 @@ void WX_VIEW_CONTROLS::onWheel( wxMouseEvent& aEvent )
 {
     const double wheelPanSpeed = 0.001;
 
+#ifdef __WXGTK3__
+    if( aEvent.GetTimestamp() == m_lastTimestamp )
+    {
+        aEvent.Skip( false );
+        return;
+    }
+
+    m_lastTimestamp = aEvent.GetTimestamp();
+#endif
+
     // mousewheelpan disabled:
     //      wheel + ctrl    -> horizontal scrolling;
     //      wheel + shift   -> vertical scrolling;

include/view/wx_view_controls.h

@@ -159,6 +159,9 @@ private:
     /// Current scrollbar position
     VECTOR2I    m_scrollPos;
 
+    /// Last event timestamp to remove duplicates
+    long int    m_lastTimestamp;
+
     /// Current cursor position (world coordinates)
     VECTOR2D    m_cursorPos;