Port of FS245 - POVMS message queue can fill up with GB of data for very fast renders

[wfpokorny]

http://bugs.povray.org/task/245 (Thorsten Fröhlich (thorsten))

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Details:

With very fast renders and very large output files, the message queue can fill up because the producers are not limited by IO, while the consumer performance is limited by disk IO. Consequently, the message queue can fill up to exhaust all available memory. The solution is to build in some better control of pending output data in the message queue on the producer side. This will also pave the way for message communication over slow links (i.e. a network).