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settings and most of the old compression control ABIs have been replaced by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/symbols.def

@@ -53,7 +53,6 @@ EXPORTS
  png_set_strip_16 @48
  png_set_quantize @49
  png_set_gamma @50
- png_set_flush @51
  png_write_flush @52
  png_start_read_image @53
  png_read_update_info @54
@@ -68,14 +67,6 @@ EXPORTS
  png_destroy_info_struct @63
  png_destroy_read_struct @64
  png_destroy_write_struct @65
- png_set_crc_action @66
- png_set_filter @67
- png_set_filter_heuristics @68
- png_set_compression_level @69
- png_set_compression_mem_level @70
- png_set_compression_strategy @71
- png_set_compression_window_bits @72
- png_set_compression_method @73
  png_init_io @74
  png_set_error_fn @75
  png_get_error_ptr @76
@@ -108,7 +99,6 @@ EXPORTS
  png_chunk_warning @106
  png_benign_error @107
  png_chunk_benign_error @108
- png_set_benign_errors @109
  png_get_valid @110
  png_get_rowbytes @111
  png_get_rows @112
@@ -184,7 +174,6 @@ EXPORTS
  png_get_header_version @182
  png_get_libpng_ver @183
  png_permit_mng_features @184
- png_set_strip_error_numbers @185
  png_set_user_limits @186
  png_get_user_width_max @187
  png_get_user_height_max @188
@@ -206,7 +195,6 @@ EXPORTS
  png_save_uint_32 @205
  png_save_uint_16 @207
  png_set_gamma_fixed @208
- png_set_filter_heuristics_fixed @209
  png_get_pixel_aspect_ratio_fixed @210
  png_get_x_offset_inches_fixed @211
  png_get_y_offset_inches_fixed @212
@@ -219,11 +207,6 @@ EXPORTS
  png_process_data_pause @219
  png_process_data_skip @220
  png_set_expand_16 @221
- png_set_text_compression_level @222
- png_set_text_compression_mem_level @223
- png_set_text_compression_strategy @224
- png_set_text_compression_window_bits @225
- png_set_text_compression_method @226
  png_set_alpha_mode @227
  png_set_alpha_mode_fixed @228
  png_set_scale_16 @229
@@ -239,9 +222,7 @@ EXPORTS
  png_image_write_to_file @239
  png_image_write_to_stdio @240
  png_convert_to_rfc1123_buffer @241
- png_set_check_for_invalid_index @242
  png_get_palette_max @243
- png_set_option @244
  png_image_write_to_memory @245
  png_memory_format @246
  png_memory_channel_depth @247