overlapped section, for improved compression

Sections 2+ read a bit of data from previous section
in order to improve compression ratio.
This also costs some CPU, to reference read data.

Read data is currently fixed to window>>3 size

lib/compress/zstdmt_compress.h

@@ -59,4 +59,4 @@ typedef enum { ZSTDMT_p_sectionSize    /* size of input "section". Each section
  * The function must be called typically after ZSTD_createCCtx().
  * Parameters not explicitly reset by ZSTDMT_init*() remain the same in consecutive compression sessions.
  * @return : 0, or an error code (which can be tested using ZSTD_isError()) */
-ZSTDLIB_API unsigned ZSTDMT_setMTCtxParameter(ZSTDMT_CCtx* mtctx, ZSDTMT_parameter parameter, unsigned value);
+ZSTDLIB_API size_t ZSTDMT_setMTCtxParameter(ZSTDMT_CCtx* mtctx, ZSDTMT_parameter parameter, unsigned value);