cea608overlay: expose timeout property

C.9 Automatic Caption Erasure (Preferred)

[...]

Some manufacturers have suggested building automatic timeout into their decoders. They propose that if no data are received for the selected caption channel within a given time, the decoder should automatically erase the caption. Such erasure may supersede the intentions of the caption service providers and institute one maximum display time for all captioning services.

If such a timeout is deemed necessary, however, the time limit should be no less than 16 seconds, an amount of time said by caption service providers to be longer than their most enduring caption. It is preferred, when automatic caption erasure is used in a decoder, that only displayed memory be erased, since some caption service providers may, contrary to recommended practice (see Section B.8.3), send pop-on style caption data to non-displayed memory more than 16 seconds before sending the EOC command which causes the caption to display.