No real changes, just a micro optimization in wxOSX DoGetAsBitmap().

Don't call wxWindow::GetSize() unnecessarily when we use the given fixed
rectangle anyhow.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76957 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775