[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/audio_device_interface.hpp

@@ -0,0 +1,36 @@
+#pragma once
+#include <array>
+
+#include "config.hpp"
+#include "helpers.hpp"
+#include "ring_buffer.hpp"
+
+class AudioDeviceInterface {
+  protected:
+	static constexpr usize maxFrameCount = 0x2000;
+
+	using Samples = Common::RingBuffer<s16, maxFrameCount * 2>;
+	using RenderBatchCallback = usize (*)(const s16*, usize);
+
+	Samples* samples = nullptr;
+
+	const AudioDeviceConfig& audioSettings;
+	// Store the last stereo sample we output. We play this when underruning to avoid pops.
+	std::array<s16, 2> lastStereoSample{};
+
+  public:
+	AudioDeviceInterface(Samples* samples, const AudioDeviceConfig& audioSettings) : samples(samples), audioSettings(audioSettings) {}
+
+	bool running = false;
+	Samples* getSamples() { return samples; }
+
+	// If safe is on, we create a null audio device
+	virtual void init(Samples& samples, bool safe = false) = 0;
+	virtual void close() = 0;
+
+	virtual void start() = 0;
+	virtual void stop() = 0;
+
+	// Only used for audio devices that render multiple audio frames in one go, eg the libretro audio device.
+	virtual void renderBatch(RenderBatchCallback callback) {}
+};