Made device to logical and vv conversion methods

  Made device to logical and vv conversion methods
    virtual and implement them like any other method
    in the wxDC classes, not only in the their files
    but somehow belonging to the wxDCBase class.
  This is required for plugging in differnt DC backends
    with different conversions.
  Ideally (and absolutely possible ) all these methods
    should be removed and just the one in wxDCBase
    should stay using values set in the various
    derived classes.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@45741 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775