Refine Nal length related structure

reviewed on https://rbcommons.com/s/OpenH264/r/434/

This refine is to remove slice number limitation in the future. The changes contains:
1. add pNalLen in Structure SWelsEncoderOutput to store each nal length
2. rename iNalLengthInByte[MAX_NAL_UNITS_IN_LAYER] to pNalLengthInByte in Structure SLayerBSInfo, the pointer point to pNalLen, like pBSBuf point to pFrameBS.

Further modification will be done to remove slice number limitation in dynamic slice mode.