[libpng16] Exit loop over tRNS array when a transparent entry is

found (performance report by PoChun Chang)
[libpng16] Change some hyphens to minus signs and fix other typos in libpng.3
[libpng16] Use minus signs, not hyphens, in "--i".
[libpng16] Mention png_set_option() in the documentation.
[libpng16] Delay calling png_init_filter_functions() until a row with nonzero

filter is found.
[libpng16] Imported from libpng-1.6.4beta01.tar
[libpng16] Bump version to 1.6.4beta02