Fix over aggressive clipping in generic wxListCtrl header drawing.

Fix over aggressive clipping in generic wxListCtrl header drawing.

Clipping out 4 pixels vertically resulted in truncating any letters with
descent (e.g. "g" or "q") under OS X where the native header size is just tall
enough to show the text.

Simply don't clip that much but use the entire header width.

Closes #11780.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63947 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775