[libpng17] Check for info_ptr == NULL early in png_read_end() so we don't need

to run all the png_handle_*() and depend on them to return if info_ptr == NULL.
This improves the performance of png_read_end(png_ptr, NULL) and makes
it more robust against future programming errors.
[libpng17] Check for __has_extension before using it in pngconf.h, to

support older Clang versions (Jeremy Sequoia).
[libpng17] Use a user warning handler in contrib/gregbook/readpng2.c instead

of default, so warnings will be put on stderr even if libpng has CONSOLE_IO
disabled.
[libpng17] Added png_ptr->process_mode = PNG_READ_IDAT_MODE in

png_push_read_chunk after recognizing the IDAT chunk, which avoids an
infinite loop while reading a datastream whose first IDAT chunk is of
zero-length.
[libpng17] Document how to use different crc_action after reading the image

data in the sequential reader.
[libpng17] Moved some documentation from png.h to libpng.3 and libpng-manual.txt

Minor editing of contrib/arm-neon/README and contrib/examples/*.c
[libpng17] Don't recognize known sRGB profiles as sRGB if they have been hacked,

but don't reject them and don't issue a copyright violation warning.
[libpng17] Imported from libpng-1.7.0beta32.tar
[libpng17] Bump version to 1.7.0beta33
[libpng17] Fixed typos in the manual and in scripts/pnglibconf.dfa

(CFLAGS -> CPPFLAGS and PNG_USR_CONFIG -> PNG_USER_CONFIG).
[libpng17] Imported from libpng-1.7.0beta33.tar
[libpng17] Bump version to 1.7.0beta34
[libpng17] Treat CRC error handling with png_set_crc_action(), instead of with

png_set_benign_errors(), which has been the case since libpng-1.6.0beta18.
[libpng17] Remove CALLOC_SUPPORTED from scripts/pnglibconf.dfa and prebuilt
[libpng17] Use "if (value != 0)" instead of "if (value)" consistently.
[libpng17] Changed ZlibSrcDir from 1.2.5 to 1.2.8 in projects/vstudio.
[libpng17] Moved configuration information from the manual to the INSTALL

file.
[libpng17] Imported from libpng-1.7.0beta34.tar
[libpng17] Bump version to 1.7.0beta35
[libpng17] Mention scripts/pnglibconf.h.prebuilt in INSTALL.