[WIP] Libretro: Add audio support (#714)

* Libretro: Add audio support

* Adding audio interface part 1

* Audio device pt 2

* More audio device

* More audio device

* Morea uudi odevice

* More audio device

* More audio device

* More audio device

---------

Co-authored-by: wheremyfoodat <44909372+wheremyfoodat@users.noreply.github.com>

include/audio/libretro_audio_device.hpp

@@ -0,0 +1,60 @@
+#pragma once
+#include <cstring>
+
+#include "audio/audio_device_interface.hpp"
+
+class LibretroAudioDevice : public AudioDeviceInterface {
+	bool initialized = false;
+
+  public:
+	LibretroAudioDevice(const AudioDeviceConfig& audioSettings) : AudioDeviceInterface(nullptr, audioSettings), initialized(false) {
+		running = false;
+	}
+
+	void init(Samples& samples, bool safe = false) override {
+		this->samples = &samples;
+
+		initialized = true;
+		running = false;
+	}
+
+	void close() override {
+		initialized = false;
+		running = false;
+	};
+
+	void start() override { running = true; }
+	void stop() override { running = false; };
+
+	void renderBatch(RenderBatchCallback callback) override {
+		if (running) {
+			static constexpr usize frameCount = 774;
+			static constexpr usize channelCount = 2;
+			static s16 audioBuffer[frameCount * channelCount];
+
+			usize samplesWritten = 0;
+			samplesWritten += samples->pop(audioBuffer, frameCount * channelCount);
+
+			// Get the last sample for underrun handling
+			if (samplesWritten != 0) {
+				std::memcpy(&lastStereoSample[0], &audioBuffer[(samplesWritten - 1) * 2], sizeof(lastStereoSample));
+			}
+
+			// If underruning, copy the last output sample
+			{
+				s16* pointer = &audioBuffer[samplesWritten * 2];
+				s16 l = lastStereoSample[0];
+				s16 r = lastStereoSample[1];
+
+				for (usize i = samplesWritten; i < frameCount; i++) {
+					*pointer++ = l;
+					*pointer++ = r;
+				}
+			}
+
+			callback(audioBuffer, sizeof(audioBuffer) / (channelCount * sizeof(s16)));
+		}
+	}
+
+	bool isInitialized() const { return initialized; }
+};