Implement wxGraphicsContext::GetSize() for Cairo.

Implement wxGraphicsContext::GetSize() for Cairo.

As the implementation of this method is basically the same for all ports move
it to the base class itself instead of requiring the derived classes to
implement it. Now the derived classes need to fill in m_width and m_height
members instead.

Do fill them when creating wxGraphicsContext in Cairo version.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67359 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775