Update png to 1.6.2beta01

Version 1.6.2beta01 [April 14, 2013]
  Updated documentation of 1.5.x to 1.6.x changes in iCCP chunk handling.
  Fixed incorrect warning of excess deflate data. End condition - the
    warning would be produced if the end of the deflate stream wasn't read
    in the last row.  The warning is harmless.
  Corrected the test on user transform changes on read. It was in the
    png_set of the transform function, but that doesn't matter unless the
    transform function changes the rowbuf size, and that is only valid if
    transform_info is called.
  Corrected a misplaced closing bracket in contrib/libtests/pngvalid.c
    (Flavio Medeiros).
  Corrected length written to uncompressed iTXt chunks (Samuli Suominen).

(prlw1)