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.

src/tangara/tts/player.cpp

@@ -174,7 +174,7 @@ auto Player::decodeToSink(const codecs::ICodec::OutputFormat& format,
 
     // The mixin PcmBuffer should almost always be draining, so we can force
     // samples into it more aggressively than with the main music PcmBuffer.
-    while (!stereo_buf.isEmpty()) {
+    while (!stereo_buf.isEmpty() && !stream_cancelled_) {
       size_t sent = output_.send(stereo_buf.readAcquire());
       stereo_buf.readCommit(sent);
     }