gtk3 "migrating" document and drawing

[Allin Cottrell <cottrell wfu edu>]

It seems that the section of the gtk-migrating-2-to-3 doc as
distributed with gtk 3.0.0 is not up to date with the state of the
drawing code in the release. For example, it still talks about
using pixmaps in the "replacement" code sections. This is an area
where it would be very helpful to have up-to-date and precise
documentation.

Allin Cottrell