on read and write.  In libpng 1.6 the two cases can be distinguished and
considerable code cleanup, and extra error checking, is possible.  This
makes calls on the write side that have no effect be ignored with a
png_app_error(), which can be disabled in the app using
png_set_benign_errors(), and removes the spurious use of usr_channels
on the read side.

MNG files).

(only update the value when it actually increases, move test for whether
the check is wanted out of the function.

options declares 'index' as a global, causing a warning if it is used as a
local variable.  GCC 64-bit warns about assigning a (size_t) (unsigned 64-bit)
to an (int) (signed 32-bit).  MSVC, however, warns about using the
unary '-' operator on an unsigned value (even though it is well defined
by ANSI-C to be ~x+1).  The padding calculation was changed to use a
different method.  Removed the tests on png_ptr->pass.

Relocated palette-index checking function from pngrutil.c to pngtrans.c

compiler issues slightly different warnings from those issued by the
current vesions of GCC. This eliminates those warnings by
adding/removing casts and small code rewrites.

Added new "png_structrp" typedef. Because of the
way libpng works both png_info and png_struct are always accessed via a
single pointer.  This means adding C99 'restrict' to the pointer gives
the compiler some opportunity to optimize the code.  This change allows that.

Fixed some compiliation problems with scaling options.

that was introduced in libpng-1.5.2beta01.

 the namespace.  Added png_get_current_row_number and
 png_get_current_pass_number for the
 benefit of the user transform callback.
 Added png_process_data_pause and png_process_data_skip for the benefit of
 progressive readers that need to stop data processing or want to optimize
 skipping of unread data (e.g. if the reader marks a chunk to be skipped.)

prototypes for most png_get_*() functions.

that CCP writes to standard output by default (Martin Banky).

Studio 2010 (John Bowler).

depending on PNG_16BIT_SUPPORTED

and shortening problems on assignment but also a few difficult to optimize
(for GCC) loops (John Bowler).

literal pointers declare them (John Bowler).
Many APIs did not change their arguments but were not declared using
PNG_CONST.  This change corrects this.  In a few cases APIs that return
constant string literal pointers have also been changed to declare this.
Unlike the argument change this may require app changes; however the
results could never be written to (the app would crash on some platforms
where strings are not writable), so this seems advisable.