wxSplitterWindow mouse capture improvements and cleanup.

- Handle mouse-capture-lost event to abort dragging mode.
- Remember mouse and sash position on buttondown event and use them as
  absolute reference during dragging. Avoid delta values from one mousemove to
  the next as this may introduce a skew during dragging and especially when
  coordinate clipping occurs.
- Clear the requested sash position when dragging in live mode.
- Draw the tracker at correct coordinates - taking into account the width of
  the pen used to draw the tracker.
- The old code did not clearly distinguish between live vs. tracking mode in
  some places.

Closes #11076.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61615 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/generic/splitter.h

@@ -193,6 +193,9 @@ public:
     // Handles mouse events
     void OnMouseEvent(wxMouseEvent& ev);
 
+    // Aborts dragging mode
+    void OnMouseCaptureLost(wxMouseCaptureLostEvent& event);
+
     // Adjusts the panes
     void OnSize(wxSizeEvent& event);
 
@@ -275,16 +278,16 @@ protected:
     wxWindow*   m_windowOne;
     wxWindow*   m_windowTwo;
     int         m_dragMode;
-    int         m_oldX;
-    int         m_oldY;
+    int         m_oldX;         // current tracker position if not live mode
+    int         m_oldY;         // current tracker position if not live mode
     int         m_sashPosition; // Number of pixels from left or top
     double      m_sashGravity;
     int         m_sashSize;
     wxSize      m_lastSize;
     int         m_requestedSashPosition;
     int         m_sashPositionCurrent; // while dragging
-    int         m_firstX;
-    int         m_firstY;
+    wxPoint     m_ptStart;      // mouse position when dragging started
+    int         m_sashStart;    // sash position when dragging started
     int         m_minimumPaneSize;
     wxCursor    m_sashCursorWE;
     wxCursor    m_sashCursorNS;