Only overflow left-aligned cells in wxGrid

Doing it for cells using a different alignment doesn't work correctly
and it's not clear whether it really makes sense, so just don't do it at
all for now.

Closes https://github.com/wxWidgets/wxWidgets/pull/1726

src/generic/grid.cpp

@@ -6086,7 +6086,11 @@ void wxGrid::DrawGridCellArea( wxDC& dc, const wxGridCellCoordsArray& cells )
                 {
                     if (!m_table->IsEmptyCell(row + l, j))
                     {
-                        if (GetCellOverflow(row + l, j))
+                        wxGridCellAttr *attr = GetCellAttr(row + l, j);
+                        const bool canOverflow = attr->CanOverflow();
+                        attr->DecRef();
+
+                        if ( canOverflow )
                         {
                             wxGridCellCoords cell(row + l, j);
                             bool marked = false;