Write code update

Implemented better defaulting of zlib settings based on image properties.
Implemented pass-through of png_write_rows when the rows can be used directly (a
common case) optimizing the handling of previous-row buffering.

Removed the METHODICAL filter selection method and disabled the HEURISTIC one;
the first was ridiculously slow (though useful for experiments) the second
doesn't work.  Filter selection is temporarily disabled (it defaults to the
lowest numbered filter in the list; typically 'none').

New handling of compression settings (incomplete), new PNG compression level
(not yet visible in an API).

Back ported 'PNG_FAST_FILTERS' from 1.6 (in png.h).

There are minimal API changes beyond removal of the selection options.  Work is
still to be done to investigate a filter selection mechanism that is at least as
good as the previous one.

Signed-off-by: John Bowler <jbowler@acm.org>

scripts/pnglibconf.h.prebuilt

@@ -1,8 +1,7 @@
 /* libpng 1.7.0beta80 STANDARD API DEFINITION */
-
 /* pnglibconf.h - library build configuration */
 
-/* Libpng version 1.7.0beta80, March 9, 2016 */
+/* libpng version 1.7.0beta80, March 9, 2016 */
 
 /* Copyright (c) 1998-2016 Glenn Randers-Pehrson */
 
@@ -100,8 +99,7 @@
 #define PNG_READ_tRNS_SUPPORTED
 #define PNG_READ_zTXt_SUPPORTED
 #define PNG_SAVE_UNKNOWN_CHUNKS_SUPPORTED
-#define PNG_SELECT_FILTER_HEURISTICALLY_SUPPORTED
-#define PNG_SELECT_FILTER_METHODICALLY_SUPPORTED
+/*#undef PNG_SELECT_FILTER_SUPPORTED*/
 #define PNG_SEQUENTIAL_READ_SUPPORTED
 #define PNG_SETJMP_SUPPORTED
 #define PNG_SETTING_SUPPORTED
@@ -193,6 +191,8 @@
 /* settings */
 #define PNG_ABORT { (abort()); }
 #define PNG_API_RULE 0
+#define PNG_COMPRESSION_BUFFER_LIMIT 8453377
+#define PNG_COMPRESSION_BUFFER_MAX 8453377
 #define PNG_DEFAULT_GAMMA_ACCURACY 665
 #define PNG_DEFAULT_READ_MACROS 1
 #define PNG_GAMMA_THRESHOLD_FIXED 153