fix the start_offset for H264BitstreamParser::IsFirstSliceInFrame while parsing with H264::NaluType::kIdr or H264::NaluType::kSlice in a stap-a packet.
In absl::optional ProcessStapAOrSingleNalu function, start_offset steps over nalu_type field.
start_offset += H264::kNaluTypeSize;
In H264BitstreamParser::IsFirstSliceInFrame, slice_reader steps over nalu_type field again.
optimize the is_real_first_packet_in_frame assignment, remove duplicated bitstream parsing.
This PR mainly does tow things.
In absl::optional ProcessStapAOrSingleNalu function, start_offset steps over nalu_type field.
In H264BitstreamParser::IsFirstSliceInFrame, slice_reader steps over nalu_type field again.