Fix order of radial gradient stops in wxMSW.

Ensure that the order of stops is consistent with the documentation and other
platforms behaviour, i.e. they are counted from inside to outside of the
circle and not vice versa, which happens to be the GDI+ convention.

Closes #16443.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@77482 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775