decoder/vorbis: scale and clip tremor-decoded samples to 15 bits

Tremor decoder is unusable since commit 2ee43c4. Sound is distorted to
the point where it's nothing but noise.

The data from vorbis_synthesis_pcmout() needs to be scaled and
clipped for 16 bit sample size. For reference see
http://lists.xiph.org/pipermail/tremor/2010-April/001642.html and
http://lists.xiph.org/pipermail/vorbis/2006-October/026513.html.

Signed-off-by: Marcin Jurkowski <marcin1j@gmail.com>

NEWS

@@ -1,6 +1,8 @@
 ver 0.20.12 (not yet released)
 * input
   - curl: fix seeking
+* decoder
+  - vorbis: fix Tremor support
 * player
   - log message when decoder is too slow
 

src/decoder/plugins/VorbisDecoderPlugin.cxx

@@ -178,6 +178,20 @@ VorbisDecoder::SubmitInit()
 	client.Ready(audio_format, eos_granulepos > 0, duration);
 }
 
+#ifdef HAVE_TREMOR
+static inline int16_t tremor_clip_sample(int32_t x)
+{
+	x >>= 9;
+
+	if (x < INT16_MIN)
+		return INT16_MIN;
+	if (x > INT16_MAX)
+		return INT16_MAX;
+
+	return x;
+}
+#endif
+
 bool
 VorbisDecoder::SubmitSomePcm()
 {
@@ -197,7 +211,7 @@ VorbisDecoder::SubmitSomePcm()
 		auto *dest = &buffer[c];
 
 		for (size_t i = 0; i < n_frames; ++i) {
-			*dest = *src++;
+			*dest = tremor_clip_sample(*src++);
 			dest += channels;
 		}
 	}