[APU, SDL] Refactor sample submission

- Move sample conversion to SDL callback thread
- Add early channel down-conversion

src/xenia/apu/conversion.h

@@ -0,0 +1,53 @@
+/**
+ ******************************************************************************
+ * Xenia : Xbox 360 Emulator Research Project                                 *
+ ******************************************************************************
+ * Copyright 2021 Ben Vanik. All rights reserved.                             *
+ * Released under the BSD license - see LICENSE in the root for more details. *
+ ******************************************************************************
+ */
+
+#ifndef XENIA_APU_CONVERSION_H_
+#define XENIA_APU_CONVERSION_H_
+
+#include <cstdint>
+
+#include "xenia/base/byte_order.h"
+
+namespace xe {
+namespace apu {
+namespace conversion {
+
+inline void sequential_6_BE_to_interleaved_6_LE(float* output,
+                                                const float* input,
+                                                size_t ch_sample_count) {
+  for (size_t sample = 0; sample < ch_sample_count; sample++) {
+    for (size_t channel = 0; channel < 6; channel++) {
+      output[sample * 6 + channel] =
+          xe::byte_swap(input[channel * ch_sample_count + sample]);
+    }
+  }
+}
+inline void sequential_6_BE_to_interleaved_2_LE(float* output,
+                                                const float* input,
+                                                size_t ch_sample_count) {
+  // Default 5.1 channel mapping is fl, fr, fc, lf, bl, br
+  // https://docs.microsoft.com/en-us/windows/win32/xaudio2/xaudio2-default-channel-mapping
+  for (size_t sample = 0; sample < ch_sample_count; sample++) {
+    // put center on left and right, discard low frequency
+    float fl = xe::byte_swap(input[0 * ch_sample_count + sample]);
+    float fr = xe::byte_swap(input[1 * ch_sample_count + sample]);
+    float fc = xe::byte_swap(input[2 * ch_sample_count + sample]);
+    float br = xe::byte_swap(input[4 * ch_sample_count + sample]);
+    float bl = xe::byte_swap(input[5 * ch_sample_count + sample]);
+    float center_halved = fc * 0.5f;
+    output[sample * 2] = (fl + bl + center_halved) * (1.0f / 2.5f);
+    output[sample * 2 + 1] = (fr + br + center_halved) * (1.0f / 2.5f);
+  }
+}
+
+}  // namespace conversion
+}  // namespace apu
+}  // namespace xe
+
+#endif