Use initRT() when setting up realtime buffers in starting()

[matthew-reynolds]

As per title. See #504 (comment).

I need to look at this again when it's not 2AM to make sure this change is safe, but I believe many of the places where we are getting a data ptr with readFromRT() and then using that to write the internal RT buffer should be replaced with initRT() to set up the realtime buffer.