fixing PNG images

[Werner LEMBERG]

I've just been bitten by the following libpng `feature':

  IDAT: invalid distance too far back

(file `lily-ledger.png', while being processed by pdftex).
Apparently, libpng 1.6.x no longer accepts PNG images with this
problem out of the box.

Note that libpng comes with a tool called `png-fix-too-far-back' to
fix such errors in PNG files.  Is it OK to update all PNG files in the
lilypond repository?  I've been able to successfully continue
compilation with the current SVN TeXlive binaries after running this
program on all PNG images.


    Werner