[libpng17] Exit loop over tRNS array once a transparent pixel is found

(performance report by PoChun Chang).
[libpng17] Change some hyphens to minus signs and fix other typos in libpng.3
[libpng17] Use minus signs, not hyphens, in "--i" in libpng.3
[libpng17] Mention png_set_option() in the manual.
[libpng17] Delay calling png_init_filter_functions() until a row with nonzero

filter is found.
[libpng17] Update comment in png_init_filter_functions()
[libpng17] Imported from libpng-1.7.0beta17.tar
[libpng17] Bump version to 1.7.0beta18