Get rid of IsOk() accessor in Cairo implementation code.

This was quite useless as m_ok flag was public anyhow, just use it directly.

Incidentally closes #16560.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77709 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775