Implemented proper slice parameter buffer submission for multiple slice streams. (#226)

* * rocDecode: Implemented proper slice parameter buffer submission for multiple slice streams.
 - We were submitting single slice parameter buffer for multiple slice streams. This works for other APIs but not for VAAPI, which requires to send slice parameter buffer once per slice.
 - At step 1, all the multi-slice structures are fixed sized (256). Will make them dynamic in the next step.
 - With this change, all 135 HEVC conformance streams now pass.

* * rocDecode: Multiple slice change step 2: make the fixed size slice info structure lists dynamic.

* * rocDecode: Minor changes in comments. No functional changes.

* *rocDecode: Changed INIT_SLICE_PARAM_LIST_NUM from 256 back to 16. 256 was set during debug process.

* * rocDecode: Add suggest changes from code review.

* * rocDecode: Moved slice_params_buf_id_ initialization to class VaapiVideoDecoder declaration, as suggested by code review.

src/rocdecode/vaapi/vaapi_videodecoder.h

@@ -42,6 +42,8 @@ THE SOFTWARE.
     }\
 }
 
+#define INIT_SLICE_PARAM_LIST_NUM 16 // initial slice parameter buffer list size
+
 class VaapiVideoDecoder {
 public:
     VaapiVideoDecoder(RocDecoderCreateInfo &decoder_create_info);
@@ -64,7 +66,8 @@ private:
 
     VABufferID pic_params_buf_id_;
     VABufferID iq_matrix_buf_id_;
-    VABufferID slice_params_buf_id_;
+    std::vector<VABufferID> slice_params_buf_id_ = std::vector<VABufferID>(INIT_SLICE_PARAM_LIST_NUM, 0);
+    uint32_t num_slices_;
     VABufferID slice_data_buf_id_;
     uint32_t slice_data_buf_size_;