Add ZLIB_CONST to make the zlib API const-correct.

git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@37256 b3059339-0415-0410-9bf9-f77b7e298cf2
Better handling of out-of-range subtitle timestamps.

Fixes decoding of PGS subtitles.

git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@37257 b3059339-0415-0410-9bf9-f77b7e298cf2