libpng-1.7.0beta49 released

libpng-1.7.0beta49 has been released, to test some changes to the filter-selection procedure to use a single "try_row" buffer instead of separate "sub_row, up_row, avg_row, and paeth_row" buffers. Please try it out and report back; if all goes well I'll port it back to libpng15 and libpng16 soon.