API reduction, PNG compression level

PNG compression level setting API: this allows the various compression settings
controlling deflate, fitlering, and so on to be set via a single setting with
six values.  This is currently documented in png.h ("Write compression
settings").

Internally the compression settings have been tuned both for the overall setting
and for any specific settings made by the original APIs.

APIs to control iCCP chunk compression separately have been added.

contrib/examples/pngcp.c has been modified to accomodate the new compression
setting and to include options for separate control of iCCP chunk compression.

The new ABI, png_setting, has been modified to accomodate a wider range of
settings and most of the old compression control ABIs have been replaced by
function-like macros with the same API which call png_setting.  This is an API
check in 1.7.0 for png_setting (alone).  png_setting now handles all of
png_set_option.  This eliminates 19 ABIs at the cost of adding 1 (png_setting).

CRC and benign error checking has been updated internally to use bit-fields and
the CRC calculation skip when the CRC is not used has been improved slightly to
avoid the initialization of the CRC.  A new png_setting based API allows more
detailed control of benign error/warning messages (this may change, the internal
error handling seems too complex.)  The ERROR_NUMBERS support has been removed
with the intent of implementing proper i18n.

The memcpy-size-0 issue in png_push_fill_buffer has been fixed, with an
appropriate debug() assert if a fill for 0 bytes occurs.

Most PNG_FLAG_ values for png_struct::flags have been eliminated (as a result of
the benign error handling changes).  Only one remains.

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

scripts/pnglibconf.dfa

@@ -285,33 +285,16 @@ setting ZLIB_HEADER default <zlib.h>
 # must be made by the user)
 option SET_OPTION disabled
 
-# Run-time setting of parameters, enabled as required below
-option SETTING disabled
-
 # To support hardware specific optimizations libpng can include a hardware
 # specific header at build time, this setting records the included header:
 setting EXTENSION_HEADER
 
-# These settings configure the default compression level (0-9) and 'strategy';
-# strategy is as defined by the implementors of zlib. It describes the input
-# data and modifies the zlib parameters in an attempt to optimize the balance
-# between search and huffman encoding in the zlib algorithms.  The defaults are
-# the zlib.h defaults - the apparently recursive definition does not arise
-# because the name of the setting is prefixed by PNG_
-#
-# The TEXT values are the defaults when writing compressed text (all forms)
+# This setting controls the default zlib compression settings.  See the
+# description of the values in png.h
+setting DEFAULT_COMPRESSION_LEVEL default PNG_COMPRESSION_MEDIUM
 
 # The '@' here means to substitute the value when pnglibconf.h is built
 setting ZLIB_VERNUM default @ZLIB_VERNUM
-setting Z_DEFAULT_COMPRESSION default @Z_DEFAULT_COMPRESSION
-# By experiment even if other strategies are favored over the zlib default it
-# still comes out best for 47% of files tested; more than filtered, the next
-# best, for 38.8% of files tested.
-setting Z_DEFAULT_STRATEGY default @Z_DEFAULT_STRATEGY
-setting Z_DEFAULT_NOFILTER_STRATEGY default @Z_DEFAULT_STRATEGY
-
-setting TEXT_Z_DEFAULT_COMPRESSION default @Z_DEFAULT_COMPRESSION
-setting TEXT_Z_DEFAULT_STRATEGY default @Z_DEFAULT_STRATEGY
 
 # Define this to something that will stop, at least, the current thread; control
 # cannot proceed beyond the PNG_ABORT operation and compilation of pngerror.c is
@@ -343,7 +326,6 @@ option WRITE_INT_FUNCTIONS disabled
 #
 # WARNINGS: normally on, if off no warnings are generated
 # ERROR_TEXT: normally on, if off errors happen but there is no message
-# ERROR_NUMBERS: unimplemented feature, therefore disabled
 # BENIGN_ERRORS: support for just issuing warnings for recoverable errors
 #
 # BENIGN_READ_ERRORS:
@@ -360,7 +342,6 @@ option WRITE_INT_FUNCTIONS disabled
 
 option WARNINGS
 option ERROR_TEXT
-option ERROR_NUMBERS disabled
 
 option BENIGN_ERRORS
 option BENIGN_WRITE_ERRORS requires BENIGN_ERRORS disabled
@@ -515,7 +496,7 @@ option READ_QUANTIZE requires READ_TRANSFORMS enables TRANSFORM_MECH
 # gamma processing; and it is an enormous waste of space.  You just need to
 # remove the use of libpng APIs that depend on it.
 option READ_GAMMA requires READ_TRANSFORMS, READ_gAMA, READ_sRGB,
-   enables TRANSFORM_MECH, READ_SCALE_16_TO_8, READ_EXPAND, SETTING
+   enables TRANSFORM_MECH, READ_SCALE_16_TO_8, READ_EXPAND
 
 option READ_ALPHA_MODE requires READ_TRANSFORMS, READ_GAMMA, READ_BACKGROUND
 option READ_BACKGROUND requires READ_TRANSFORMS, READ_STRIP_ALPHA, READ_GAMMA,
@@ -852,10 +833,8 @@ option WRITE_tEXt enables WRITE_TEXT
 option WRITE_zTXt enables WRITE_TEXT
 option WRITE_iTXt enables WRITE_TEXT
 
-# This only affects support of the optional PLTE chunk in RGB and RGBA
-# images.  Notice that READ_ANCILLARY_CHUNKS therefore disables part
-# of the regular chunk reading too.
-
+# WARNING: unless this option is set PLTE chunks in non-palette images are
+# simply discarded (with checking of the CRC, but nothing else.)
 option READ_OPT_PLTE requires READ_ANCILLARY_CHUNKS
 
 # Unknown chunk handling