Make row/column drag-resizing in wxGrid "live"

Update the column width immediately, as it's being dragged, instead of
drawing a temporary line showing the new column boundary using wxINVERT.

This results in better user experience, as it the effect of changing the
column width can be immediately seen (especially important for non-left
aligned columns or columns using ellipsizition) and, equally if not more
importantly, fixes wxGrid drag-resize not showing any visible UI at all
with wxGTK3 and wxOSX where wxINVERT is not implemented.

include/wx/generic/grid.h

@@ -2302,10 +2302,11 @@ protected:
     // in progress.
     int     m_dragMoveCol;
 
-    // the last position (horizontal or vertical depending on whether the user
-    // is resizing a column or a row) where a row or column separator line was
-    // dragged by the user or -1 of there is no drag operation in progress
+    // Last horizontal mouse position while drag-moving a column.
     int     m_dragLastPos;
+
+    // Row or column (depending on m_cursorMode value) currently being resized
+    // or -1 if there is no resize operation in progress.
     int     m_dragRowOrCol;
 
     // true if a drag operation is in progress; when this is true,
@@ -2480,11 +2481,6 @@ private:
                         const wxGridCellCoords& coords,
                         bool isFirstDrag);
 
-    // process row/column resizing drag event
-    void DoGridLineDrag(int pos,
-                        const wxGridOperations& oper,
-                        wxGridWindow* gridWindow);
-
     // process mouse drag event in the grid window, return false if starting
     // dragging was vetoed by the user-defined wxEVT_GRID_CELL_BEGIN_DRAG
     // handler
@@ -2493,16 +2489,11 @@ private:
                          bool isFirstDrag,
                          wxGridWindow* gridWindow);
 
-    void DrawGridDragLine(wxPoint position,
+    // Update the width/height of the column/row being drag-resized.
+    void DoGridDragResize(const wxPoint& position,
                           const wxGridOperations& oper,
                           wxGridWindow* gridWindow);
 
-    // return the current grid windows involved in the drag process
-    void GetDragGridWindows(int pos,
-                            const wxGridOperations& oper,
-                            wxGridWindow*& firstGridWindow,
-                            wxGridWindow*& secondGridWindow);
-
     // process different clicks on grid cells
     void DoGridCellLeftDown(wxMouseEvent& event,
                             const wxGridCellCoords& coords,
@@ -2534,6 +2525,7 @@ private:
 
     void DoColHeaderClick(int col);
 
+    void DoStartResizeRowOrCol(int col);
     void DoStartMoveCol(int col);
 
     void DoEndDragResizeRow(const wxMouseEvent& event, wxGridWindow *gridWindow);