GtkImage: Drop support for storing pixbufs

GtkImage: Drop support for storing pixbufs

This drops the pixbuf property and the pixbuf getters. We keep
gtk_image_new/set_from_pixbuf, but these are small helpers that
immediately convert to a surface, and there is no way to later get
back the pixbuf you passed in.

The from file/resource codepaths are also changed to load a surface
instead of a pixbuf.