Introduce ZSTD_getFrameProgression()

Produces 3 statistics for ongoing frame compression :
- ingested
- consumed (effectively compressed)
- produced

Ingested can be larger than consumed due to buffering effect.

For the time being, this patch mostly fixes the % ratio issue,
since it computes consumed / produced,
instead of ingested / produced.

That being said, update is not "smooth",
because on a slow enough setting,
fileio spends most of its time waiting for a worker to complete its job.

This could be improved thanks to more granular flushing
i.e. start flushing before ongoing job is fully completed.

lib/compress/zstdmt_compress.h

@@ -122,6 +122,13 @@ size_t ZSTDMT_CCtxParam_setNbThreads(ZSTD_CCtx_params* params, unsigned nbThread
  * mtctx must be valid */
 unsigned ZSTDMT_getNbThreads(const ZSTDMT_CCtx* mtctx);
 
+/* ZSTDMT_getFrameProgression():
+ * tells how much data has been consumed (input) and produced (output) for current frame.
+ * able to count progression inside worker threads.
+ */
+ZSTD_frameProgression ZSTDMT_getFrameProgression(ZSTDMT_CCtx* mtctx);
+
+
 /*! ZSTDMT_initCStream_internal() :
  *  Private use only. Init streaming operation.
  *  expects params to be valid.