ZLMedia中track的建立（2）--视频

h264编码track的建立，从rtmp中sprop-parameter-sets中取出经过base64编码过后的sps和pps，经解码以后得到sps和pps,然后根据sps和pps建立track.

序列参数集（SPS）
图像参数集（PPS）

        case CodecH264 : {
            //a=fmtp:96 packetization-mode=1;profile-level-id=42C01F;sprop-parameter-sets=Z0LAH9oBQBboQAAAAwBAAAAPI8YMqA==,aM48gA==
            auto map = Parser::parseArgs(track->_fmtp, ";", "=");
            auto sps_pps = map["sprop-parameter-sets"];
            string base64_SPS = FindField(sps_pps.data(), NULL, ",");
            string base64_PPS = FindField(sps_pps.data(), ",", NULL);
            auto sps = decodeBase64(base64_SPS);
            auto pps = decodeBase64(base64_PPS);
            if (sps.empty() || pps.empty()) {
                //如果sdp里面没有sps/pps,那么可能在后续的rtp里面恢复出sps/pps
                return std::make_shared<H264Track>();
            }
            return std::make_shared<H264Track>(sps, pps, 0, 0);
        }