1. works correctly (handles negative widths according to the docs/MSW/Univ)
2. has shorter and simpler GetFieldRect() implementation
3. ... which is also more efficient (pixel widths are cached)
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@16966 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775