exported symbols in theme engines

["Matthias Clasen" <matthias clasen gmail com>]

There have been some problems recently with
carelessly exported symbols from some theme engines
leading to symbol resolution errors in some apps (see
bug 326249).

In order to help with cleaning this up, I have written a little
script to check that a theme engine only exports the expected
functions. You can find it in GTK+ HEAD, called
gtk-engine-check-abi.sh.

As a good citizen, I started by making the pixbuf engine
pass this test...

Matthias