Filter code change prep.

This commit moves code round and changes the filter write interfaces that took
png_uint_32 buffer pixel counts to unsigned int.  Also moves compression code
and definitions into pngwutil.c so that the compression code is isolated from
other definitions.

Signed-off-by: John Bowler <jbowler@acm.org>

pngread.c

@@ -751,7 +751,7 @@ png_read_destroy(png_structrp png_ptr)
 
       if (ret != Z_OK)
       {
-         png_zstream_error(png_ptr, ret);
+         png_zstream_error(&png_ptr->zstream, ret);
          png_warning(png_ptr, png_ptr->zstream.msg);
       }
    }