Turning APNG to on by default in graphics/png

[Mikhail T.]

I'd like to see this option on by default now. I think, it's been available (off 
by default) long enough and I, for one, always turn it on for completeness. 
There used to be concerns about security of animated PNG code, but today I can 
not find any advisories fresher than 2008:

    http://osvdb.org/show/osvdb/48766

Various Mozilla applications will then be able to LIB_DEPEND on the installed 
png instead of building their own versions.

Comments? Thanks!

    -mi