[libpng16] Fixed conceivable but difficult to repro overflow. Also added

two test programs to generate and test a PNG which should have the problem.
[libpng16] Imported from libpng-1.6.0beta37.tar
[libpng16] Bump version to 1.6.0beta38
[libpng16] Corrected previous attempt at overflow detection in

png_set_unknown_chunks().
[libpng16] Imported from libpng-1.6.0beta38.tar
[libpng16] Bump version to 1.6.0beta39
[libpng16] Added overflow detection in png_set_sPLT() and png_set_text_2().
[libpng16] Imported from libpng-1.6.0beta39.tar
[libpng16] Bump version to 1.6.0beta40
[libpng16] Use consistent handling of overflows in text, sPLT and unknown

png_set_* APIs
[libpng16] Push two files that weren't updated in previous push.
[libpng16] Imported from libpng-1.6.0beta40.tar
[libpng16] Bump version to 1.6.0beta41
[libpng16] Imported from libpng-1.6.0rc01.tar
[libpng16] Added documentation of the new png_get_palette_max() function.
[libpng16] Imported from libpng-1.6.0rc02.tar
[libpng16] Bump version to 1.6.0rc03
[libpng16] Fixed palette_max API.
[libpng16] Imported from libpng-1.6.0rc03.tar
[libpng16] Bump version to 1.6.0rc04