MKID_BE relied on unspecified behavior of the C++ compiler,
and as such was always a bit unsafe. The new MKTAG macro
is slightly less elegant, but does no longer depend on the
behavior of the compiler.
Inspired by FFmpeg, which has an almost identical macro.
libpng is still needed for PNG encoding (for thumbnails in saved games of sword25), but
since we'll probably drop support for the original saved games anyway, the PNG encoding
code will ultimately be removed
svn-id: r55723
