Make wxGetPangoContext() work even without open display.

Use default Pango font map if we don't have any default screen in wxGTK.

Closes #16240.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76482 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775