[libpng17] Imported from libpng-1.7.0beta15.tar
[libpng17] Bump version to 1.7.0beta16
[libpng17] Fix spelling of "doclifter" in CHANGES and ANNOUNCE
[libpng17] Revised example.c to illustrate use of PNG_DEFAULT_sRGB and

PNG_GAMMA_MAC_18 as parameters for png_set_gamma().