Include zlib.h when necessary.
Include zlib.h when necessary.

Newer versions of png.h don't include it for us.
This is only needed when compiling with OC_DUMP_IMAGES (i.e., for
 debugging).

svn path=/trunk/theora/; revision=18760

brion/theora,brion/theora,brion/theora,brion/theora,brion/theora