id_sd_sdl: Fixes to stereo support

Two fixes here:
1. Technically, it's not valid C to access *stream while incrementing it
   with *stream++. Oops!
2. Forgot a multiply by numChannels, so we were using the wrong offset
   when copying pre-rendered stereo buffers. This caused some stereo
   crackle.

The latter's the only problem, and irritatingly affects OSI v1.02

src/id_sd_sdl.c

@@ -279,9 +279,13 @@ static inline void PCSpeakerUpdateOne(int16_t *stream, int length)
 {
 	for (int loopVar = 0; loopVar < length; loopVar++)
 	{
-		*stream++ = (*stream + SD_SDL_CurrentBeepSample) / 2; // Mix
+		*stream = (*stream + SD_SDL_CurrentBeepSample) / 2; // Mix
+		stream++;
 		if (numChannels == 2)
-			*stream++ = (*stream + SD_SDL_CurrentBeepSample) / 2; // Mix
+		{
+			*stream = (*stream + SD_SDL_CurrentBeepSample) / 2; // Mix
+			stream++;
+		}
 		SD_SDL_BeepHalfCycleCounter += 2 * PC_PIT_RATE;
 		if (SD_SDL_BeepHalfCycleCounter >= SD_SDL_BeepHalfCycleCounterUpperBound)
 		{
@@ -325,7 +329,7 @@ void SD_SDL_CallBack(void *unused, Uint8 *stream, int len)
 		{
 			// Copy sound generated by alOut
 			if (SD_ALOut_SamplesEnd - SD_ALOut_SamplesStart > 0)
-				memcpy(currSamplePtr, &SD_ALOut_Samples[SD_ALOut_SamplesStart], numChannels * 2 * (SD_ALOut_SamplesEnd - SD_ALOut_SamplesStart));
+				memcpy(currSamplePtr, &SD_ALOut_Samples[SD_ALOut_SamplesStart * numChannels], numChannels * 2 * (SD_ALOut_SamplesEnd - SD_ALOut_SamplesStart));
 			// Generate what's left
 			if (currNumOfSamples - (SD_ALOut_SamplesEnd - SD_ALOut_SamplesStart) > 0)
 				YM3812UpdateOne(&oplChip, currSamplePtr + (SD_ALOut_SamplesEnd - SD_ALOut_SamplesStart) * numChannels, currNumOfSamples - (SD_ALOut_SamplesEnd - SD_ALOut_SamplesStart));