Some Pixbuf, PixbufAnimation functions on Windows is exported with "_utf8" suffix.
I hid generated versions, copied and modified code to use "_utf8" on Windows.
Also hid "gdk_pixbuf_new_from_resource*", "gdk_pixbuf_animation_new_from_resource" because they are using GResources and we already have constructors for .NET resources.
This exposes gdk_pixbuf_get_from_window and gdk_pixbuf_get_from_surface via Gdk.Pixbuf constructors. Previously they were in a hidden GdkPixbuf_ class.
Fixes: #174