Audio backend improvements

Callback based audio update.
Upgraded common backend interface.
Added Cubeb backend.
Support multiple audio providers.
Dropped pulse, alsa, openal backends.

Utilities/simple_ringbuf.h

@@ -0,0 +1,31 @@
+#pragma once
+
+#include "util/types.hpp"
+#include "util/atomic.hpp"
+
+// Single reader/writer simple ringbuffer.
+// Counters are 32-bit.
+class simple_ringbuf
+{
+private:
+
+	atomic_t<u64> rw_ptr = 0;
+	u32 buf_size = 0;
+	std::unique_ptr<u8[]> buf{};
+	atomic_t<bool> initialized = false;
+
+public:
+
+	simple_ringbuf() {};
+	simple_ringbuf(u32 size);
+
+	u32 get_free_size();
+	u32 get_used_size();
+
+	// Thread unsafe functions.
+	void set_buf_size(u32 size);
+	void flush(); // Could be safely called from reader.
+
+	u32 push(const void *data, u32 size);
+	u32 pop(void *data, u32 size);
+};