Fixing wrong left position of resizing partition

Hello Tony,

the changes of `dragStart` in the last version of jqGrid (4.5.4) uses now `clientX` property of the mouse `mousedown` event. The calculation of `left` position of the div (`div.ui-jqgrid-resize-mark`) has wrong value. The div is the child of `gbox` (sibling of `ui-jqgrid-view`) which has `position:relative;`. So one have to use *relative* position values for `left` and `top` CSS properties of `div.ui-jqgrid-resize-mark`. The suggested code uses `$(this.bDiv).offset().left` (the same as `$(this.bDiv).closest(".ui-jqgrid-view").offset().left`) instead on unclear constant `6` value from the current code of `dragStart`.

[The demo](http://www.ok-soft-gmbh.com/jqGrid/WrongPositionOfSeparaterBarDuringColumnResizing0.htm) uses

    <div style="margin-left:20px">
      <table id="list"><tr><td></td></tr></table>
      <div id="pager"></div>
    </div>

and it can be used to demonstrate that jqGrid 4.5.4 wrong calculate left position of resizing div.

[Another demo](http://www.ok-soft-gmbh.com/jqGrid/WrongPositionOfSeparaterBarDuringColumnResizing.htm) uses suggestions of the current pull request and it works correct.

Best regards
Oleg