Only png_read_finish_row() can be omitted.

which are only used by sequential PNG readers.  This can save some
resources in applications that only do progressive reading.

This reverts commit f4276c36.

having to do with sCAL chunk handling

sig_bit was not declared in write_png() and was used differently from
sig_bit in read_png(), as reported by Vincent Torri.  This update changes
sig_bit to sig_bit_p in read_png() and declares sig_bit in write_png().

The memset() should use "row_bytes" instead of "png_ptr->rowbytes".
This bug was introduced in libpng-1.2.36.

The variable "gamma" is also defined in math.h on some platforms.