Change tts:luminanceGain to non-inheritable (#715).

[skynavga]

Closes #715.

[nigelmegitt]

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[nigelmegitt]

Seems like a good change to me, given tts:luminanceGain applying only to region.

However, I wonder how it is supposed to work in practice - I might expect the value to vary in time on a subtitle by subtitle basis and therefore be applied to a p. That's a whole separate question though.

[palemieux]

However, I wonder how it is supposed to work in practice - I might expect the value to vary in time on a subtitle by subtitle basis and therefore be applied to a p. That's a whole separate question though.

Using <set> to modulate the value?

[nigelmegitt]

Using <set> to modulate the value?

OK if you're willing to take the authorial pain of having the content times completely separated from the luminanceGain times. Maybe that's okay, if the variation of luminanceGain over time can be algorithmically calculated base only on some processing of the video without reference to the subtitle text.