Fix wxRegion::Offset() copy on write behaviour in wxMSW.

Offset the correct, new and unshared, region handle instead of the old and
possibly shared one.

Closes #15690.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75249 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775