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.

Revised contrib/gregbook to work under cygwin/mingw.

they are no longer used.

  The changes include support for function attributes in VC in addition to
    those already present in GCC - necessary because without these some
    warnings are unavoidable.  Fixes include signed/unsigned fixes in
    pngvalid and checks with gcc -Wall -Wextra -Wunused.
  VC requires function attributes on function definitions as well as
    declarations, PNG_FUNCTION has been added to enable this and the
    relevant function definitions changed.

that were recently introduced, mostly resulting from
safe but uncast assignments to shorter integers.  Also added a zlib
VStudio release library project because the latest zlib Official Windows
build does not include such a thing.

Renamed "png_sucker" to "png_store" in pngvalid.c

Also png_set_filter_heuristics_fixed and png_get_aspect_ratio_fixed.