Merge libpng 1.6.2 into the trunk.

Add pngprefix.h file generated by libpng configure/make process (which we
don't run during wxWidgets build, so this is something that will need to be
redone manually whenever libpng is upgraded, the procedure for doing this is
documented in docs/tech/tn0025.txt) in order to use "wx_" prefix for all
libpng public symbols. This should avoid at least some problems due to the
conflicts between the built-in libpng and shared libpng loaded into the
process as GTK+ libraries dependency under Unix.

Also rename wx_png_{warning,error} to use upper case PNG for consistency with
wx_PNG_stream_{reader,writer} and to avoid conflict with png_{warning,error}()
which we now rename to have "wx_" prefix.

Closes #14157.


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74451 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775