about using png_set_io_ptr() in configure scripts for detecting libpng

to add PNG_NO_READ_SWAP, PNG_NO_WRITE_SWAP

and "else statement;" into two lines.

Because of many cosmetic changes, drop back to beta and do rc01 later.

Evidently the fix didn't go into the real master copy.

"pngminus" should be "gregbook"

(README, gather.sh, makefile.std, and pngusr.h)

also minor editing of encoder/pngusr.h and decoder/pngusr.h

Only png_read_finish_row() can be omitted.

This stuff should be handled in the APNG fork.

This should be handled in the APNG fork, not in libpng proper.

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

Added advice about setting pointers to NULL after png_free(),
capitalized sentences, reformatted long comments in libpng style.

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.

These are mostly just cosmetic changes.

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

Changed some local variable names such as "buffer" to "buff".