ported from libpng-1.4.0

from 1.2.41beta05, 1.2.41beta11, and 1.2.41beta12.

due to popular demand.

as an aid to migration to 1.4.x

to support WindowsCE.

Revised iTXt support. To ensure binary compatibility with
previous versions, the "lang" and "lang_key" members will be assumed
to be omitted from versions prior to 1.2.41beta11 whenever there is a
library mismatch.

(inside an #if 0/#endif block)

and avoid reference to time.h "tm" structure in png.h and pngwrite.c

    To ensure binary compatibility with
    previous versions, the "lang" and "lang_key" members will be assumed
    to be omitted from previous versions unless the current libpng
    version was built with PNG_iTXt_SUPPORTED (which is otherwise no
    longer necessary to gain iTXt support), as a signal that the user has
    been building previous versions with PNG_iTXt_SUPPORTED as well.

Merged libpng-1.2.41.txt with libpng-1.4.0.txt where appropriate.

into the main libpng directory.  More bugfixes and improvements to
CMakeLists.txt (Philip Lowman)

from pngwrite.c

and png_set_user_cache_max() from libpng-1.4.0rc01.