Fix referencing of cairo_t returned from wxDCImpl::GetCairoContext().
It was assumed that all callers were going to unref the cairo_t, but that is not
true, so callers that are going to unref it should ref it themselves.
See #15455


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74746 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775