Preserve wxSlider value when changing its range in wxOSX too.

Preserve wxSlider value when changing its range in wxOSX too.

This fix is similar to r66368 for wxMSW and preserves the slider value when
its range changes. This is necessary because while the underlying native
control value doesn't change when the range is, the logical value of wxSlider
does change if it must be inversed as this depends on the range.

See #12765.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66399 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775