Support snd resources with non-0 sustain loop start frame

src/SoundMixer/SoundManager.cpp

@@ -175,8 +175,21 @@ static void InstallSoundInChannel(SndChannelPtr chan, const Ptr sampledSoundHead
 	{
 		impl.source.SetLoop(true);
 
-		if (info.loopStart != 0)
-			TODO2("Warning: looping on a portion of the snd isn't supported yet");
+		// Set sustain loop start frame
+		if ((int) info.loopStart >= impl.source.length)
+		{
+			TODO2("Warning: Illegal sustain loop start frame");
+		}
+		else
+		{
+			impl.source.sustainOffset = info.loopStart;
+		}
+
+		// Check sustain loop end frame
+		if ((int) info.loopEnd != impl.source.length)
+		{
+			TODO2("Warning: Unsupported sustain loop end frame");
+		}
 	}
 
 	//---------------------------------

src/SoundMixer/cmixer.cpp

@@ -247,6 +247,7 @@ void Source::Init(int theSampleRate, int theLength)
 {
 	this->samplerate = theSampleRate;
 	this->length = theLength;
+	this->sustainOffset = 0;
 	SetGain(1);
 	SetPan(0);
 	SetPitch(1);
@@ -592,7 +593,7 @@ void WavStream::FillBuffer(int16_t* dst, int fillLength)
 		// Loop back and continue filling buffer if we didn't fill the buffer
 		if (fillLength > 0)
 		{
-			idx = 0;
+			idx = sustainOffset;
 		}
 	}
 }

src/SoundMixer/cmixer.h

@@ -46,6 +46,7 @@ namespace cmixer
 		int16_t pcmbuf[BUFFER_SIZE];    // Internal buffer with raw stereo PCM
 		int samplerate;                 // Stream's native samplerate
 		int length;                     // Stream's length in frames
+		int sustainOffset;              // Offset of the sustain loop in frames
 		int end;                        // End index for the current play-through
 		int state;                      // Current state (playing|paused|stopped)
 		int64_t position;               // Current playhead position (fixed point)