revid: reconsider how lexTo works when we stop input

[Saxon Milton]

Currently we rely on the Lex functions to terminate once they get an EOF, but this is an unreliable methodology given that some inputs might not provide an EOF when they stop. For example, manual input, which describes the process of manually providing the revid pipeline with frames, would not provide an EOF when input is stopped, there would just cease to be any writes to the pipeline.

As such, it’s proposed that we add another parameter to the lex signature: a signal chan to signal when we should terminate lexing.