[libpng15] Bail out of loop over tRNS array once a transparent entry

is found.
[libpng15] Change some hyphens to minus signs and fix other typos in libpng.3
[libpng15] Mention png_set_option() in the documentation.
[libpng15] Delay calling png_init_filter_functions() until a row with nonzero

filter is found.
[libpng15] Imported from libpng-1.5.18beta01.tar
[libpng15] Bump version to 1.5.18beta02