Correct errors in TransformPoint() and TransformDistance().
Change Rotate() to interpret positive angles as rotating clockwise, for
consistency with wxGraphicsContext::Rotate().
Improve the unit test to verify that all the transformations work correctly.
Closes#14334.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71555 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775