how to pub protobuf data in zero copy with shm

[yeyueluke]

hi:
in the example of z_bytes.cxx, it shows how to serialize protobuf data into Bytes,
I want to know how to pub the protobuf data in shm with zero copy,
if do as in z_pub_shm.cxx, it should memcpy protobuf data to shm,
if protobuf data is very big, eg 4MB lidar points, this is really cost time
so how to to use zero copy with large protobuf data.

thx