Log Message:
Update to 1.2.27beta3, fixing a possible application security problem.
See http://libpng.sourceforge.net/Advisory-1.2.26.txt for details.

Changes since 1.2.26:
version 1.2.27beta01 [April 12, 2008]
  Fixed bug (introduced in libpng-1.0.5h) with handling zero-length
    unknown chunks.
  Added more information about png_set_keep_unknown_chunks() to the
    documetation.
  Reject tRNS chunk with out-of-range samples instead of masking off
    the invalid high bits as done in since libpng-1.2.19beta5.

version 1.2.27beta02 [April 13, 2008]
  Revised documentation about unknown chunk and user chunk handling.
  Keep tRNS chunk with out-of-range samples and issue a png_warning().

version 1.2.27beta03 [April 14, 2008]
  Added check for NULL ptr in TURBOC version of png_free_default().
  Removed several unnecessary checks for NULL before calling png_free().
  Revised png_set_tRNS() so that calling it twice removes and invalidates
    the previous call.
  Revised pngtest to check for out-of-range tRNS samples.