Remove the return value from NativeMix, correct mixing with other channel numbers than 2

UI/NativeApp.cpp

@@ -803,19 +803,27 @@ void NativeInit(int argc, const char *argv[], const char *savegame_dir, const ch
 void CallbackPostRender(UIContext *dc, void *userdata);
 bool CreateGlobalPipelines();
 
+// TODO: Add faster special case for channels == 2.
 void NativeMixWrapper(float *dest, int framesToWrite, int channels, int sampleRateHz, void *userdata) {
 	static int16_t *buffer;
 	static int bufSize;
-	if (bufSize < framesToWrite * channels) {
-		buffer = new int16_t[framesToWrite * channels];
-		bufSize = framesToWrite * channels;
+	if (bufSize < framesToWrite * 2) {
+		buffer = new int16_t[framesToWrite * 2];
+		bufSize = framesToWrite * 2;
 	}
 
 	NativeMix(buffer, framesToWrite, sampleRateHz, userdata);
 
-	for (int i = 0; i < framesToWrite * channels; i++) {
-		int16_t value = buffer[i];
-		dest[i] = (float)value * (float)(1.0f / 32767.0f);
+	for (int i = 0; i < framesToWrite; i++) {
+		int16_t lvalue = buffer[i * 2];
+		int16_t rvalue = buffer[i * 2 + 1];
+		dest[i * channels] = (float)lvalue * (float)(1.0f / 32767.0f);
+		if (channels > 1) {
+			dest[i * channels + 1] = (float)lvalue * (float)(1.0f / 32767.0f);
+		}
+		for (int j = 2; j < channels; j++) {
+			dest[i * channels + j] = 0.0f;
+		}
 	}
 }