[tex-live] please update to libpng 1.6.2

[Karl Berry]

No chance to read any follow-ups, but wanted to post this idea that my
brain was mulling over last night ...

- proceed with the DVD release as planned, freezing next week with the
current binaries.  (Barring other disasters.)

- but expect to make a bug-fix (only) release of pdftex/mpost/whatever
after there has been enough time to fix (and test).  As we did with
the xetex bug-fix binary some years ago, it would be done off a branch.

- as long we restrict the changes to the affected binaries, it would be
easy for anyone to get them.  They wouldn't have to run tlmgr or get all
the other unrelated changes to the packages, but could just download the
new binaries (by hand) and put them in place (by hand).  E.g., wget && mv.

It's the best compromise I've been able to come up with.

k