[devel] Update README files.
[devel] Update CHANGES and ANNOUNCE
[devel] Bump to version libpng-1.5.0beta18
[devel] Note in CHANGES that pngusr.h is unuseable

and fixed a small typo in a comment in pngconf.h
[devel] Change DITHER to QUANTIZE in the pnglibconf.h maintenance scripts
[devel] Attempt to get contrib/pngminim working with new pnglibconf.h

not quite working yet (pow is undefined)
[devel] Revised contrib/pngminim/decoder to build and use pnglibconf.h
[devel] Restored the ability to include optional pngusr.h
[devel] Moved replacements for png_error() and png_warning() from the

    contrib/pngminim project to pngerror.c, for use when warnings or
    errors are disabled via PNG_NO_WARN or PNG_NO_ERROR_TEXT, to avoid
    storing unneeded error/warning text.
[devel] Updated contrib/pngminim project to work with the new pnglibconf.h
[devel] Added some PNG_NO_* defines to contrib/pngminim/*/pngusr.h to

save some space.
[devel] Imported from libpng-1.5.0beta18.tar
[devel] Bump to version libpng-1.5.0beta19
[devel] Fix 'make distcheck'
[devel] Update Makefile.in to delete the xcode project.
[devel] Don't force QUANTIZE on in scripts/symbols.dfn

It's now already on by default.
[devel] Added PNG_{READ,WRITE}_INT_FUNCTIONS_SUPPORTED.

This allows the functions
to read and write ints to be disabled independently of PNG_USE_READ_MACROS,
which allows libpng to be built with the functions even though the default
is to use the macros - this allows applications to choose at app build
time whether or not to use macros (previously impossible because the
functions weren't in the default build.)
[devel] Changed Windows calling convention back to __cdecl for API functions.

For Windows/x86 platforms only:
  __stdcall is no longer needed for Visual Basic, so libpng-1.5.0 uses
  __cdecl throughout (both API functions and callbacks) on Windows/x86
  platforms.
[devel] Replaced visualc6 and visualc71 projects with new vstudio project
[devel] Imported from libpng-1.5.0beta19.tar