for GTK+ 3.6 and later, invalidate cached best size when GTK's style cache is updated, see #16088


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76149 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775