In Windows builds of GTK+ 3.x, the dll filename for GDK is
libgdk-3-0.dll.
We use this opportunity to use a common const in the DllImport for all
custom code.
* cairo/Surface.cs: expose LookupSurface
* cairo/cairo-api.xml: add surface mapping
* gdk/*: updates and hobbling to get gdk building. lots of work
to come.