TTS: Avoid exhausting the WorkerPool with concurrent TTS playback.

Reported in issue #258.

As of v1.2.0, if /.tangara-tts/ samples are present on the SD card, and
>= 4 menu items with matching TTS samples are highlighted in the UI, and
no audio output (headphones or BT sink) is connected, the `tts::Player`'s
invocation of lambdas on the WorkerPool will result in worker task
exhaustion.

This is because we get stuck in state where the `drivers::PcmBuffer` is
not accepting any new samples, and the inner loop in `Player::decodeToSink`
that pushes to the output isn't checking to see whether playback was
cancelled. So the loop never terminates, and we consume that worker slot.

Repeat with another 3 menu items, and, hey, all four worker threads are
consumed with TTS that will not terminate until headphones/BT are connected.

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