Perf sample: Added decoded frame number and display delay options to video decode performance sample. (#405)

* * rocDecode/Perf sample: Added decoded frame number and display delay options to video decode performance sample. Also changed default thread number from 4 to 1.

* * rocDecode: Added max number of decoded frames option to decode sample script. This is useful to do partial decoding test on long streams.

* * rocDecode: Updated README.md.

* * rocDecode: Minor correction.

[ROCm/rocdecode commit: c486a7f6b4]

projects/rocdecode/test/testScripts/README.md

@@ -43,7 +43,9 @@ optional arguments:
   --sample_mode SAMPLE_MODE
                         The sample to run - optional (default:0 [range:0-1] 0: videoDecode, 1: videoDecodePerf)
   --num_threads NUM_THREADS
-                        The number of threads is only for the videoDecodePerf sample (sample_mode = 1) - optional (default:4)
+                        The number of threads is only for the videoDecodePerf sample (sample_mode = 1) - optional (default:1)
+  --max_num_decoded_frames MAX_NUM_DECODED_FRAMES
+                        The max number of decoded frames. Useful for partial decoding of a long stream. - optional (default:0, meaning no limit)
 ```
 
 * **run_rocDecode_Conformance.py**