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.