[AD] retrace_proc is wrong

[Peter Wang <tjaden@xxxxxxxxxx>]

retrace_proc works just like a "timer" callback, so could be run from a 
background thread.  This means things like altering the palette should 
not be allowed, meaning the documentation is utterly out of date:

extern void (*retrace_proc)();
    [snip]
    The only use I can see for
    this function is for doing palette manipulations, because triple
    buffering can be done with the request_scroll() function, and the
    retrace_count variable can be used for timing your code. If you
    want to alter the palette in the retrace_proc you should use the
    inline _set_color() function rather than the regular set_color() or
    set_palette(), and you shouldn't try to alter more than two or
    three palette entries in a single retrace.

I suggest we deprecate retrace_proc in the unstable branch.  Given its 
uselessness, I would like to see it removed.  It could be retained as a 
DOS-only feature, but deprecated.

ex3buf should also be fixed.

Peter