Rewrite host GPU abstraction

- Don't have to repeat the same thing for 4 renderers.
 - Add native Metal renderer.

src/util/metal_stream_buffer.h

@@ -0,0 +1,65 @@
+// SPDX-FileCopyrightText: 2023 Connor McLaughlin <stenzek@gmail.com>
+// SPDX-License-Identifier: (GPL-3.0 OR CC-BY-NC-ND-4.0)
+
+#pragma once
+
+#include "common/types.h"
+
+#include <Metal/Metal.h>
+#include <QuartzCore/QuartzCore.h>
+
+#ifndef __OBJC__
+#error This file needs to be compiled with Objective C++.
+#endif
+
+#if __has_feature(objc_arc)
+#error ARC should not be enabled.
+#endif
+
+#include <deque>
+#include <memory>
+
+class MetalStreamBuffer
+{
+public:
+  MetalStreamBuffer();
+  MetalStreamBuffer(MetalStreamBuffer&& move) = delete;
+  MetalStreamBuffer(const MetalStreamBuffer&) = delete;
+  ~MetalStreamBuffer();
+
+  MetalStreamBuffer& operator=(MetalStreamBuffer&& move) = delete;
+  MetalStreamBuffer& operator=(const MetalStreamBuffer&) = delete;
+
+  ALWAYS_INLINE bool IsValid() const { return (m_buffer != nil); }
+  ALWAYS_INLINE id<MTLBuffer> GetBuffer() const { return m_buffer; }
+  ALWAYS_INLINE u8* GetHostPointer() const { return m_host_pointer; }
+  ALWAYS_INLINE u8* GetCurrentHostPointer() const { return m_host_pointer + m_current_offset; }
+  ALWAYS_INLINE u32 GetCurrentSize() const { return m_size; }
+  ALWAYS_INLINE u32 GetCurrentSpace() const { return m_current_space; }
+  ALWAYS_INLINE u32 GetCurrentOffset() const { return m_current_offset; }
+
+  bool Create(id<MTLDevice> device, u32 size);
+  void Destroy();
+
+  bool ReserveMemory(u32 num_bytes, u32 alignment);
+  void CommitMemory(u32 final_num_bytes);
+
+private:
+  bool AllocateBuffer(u32 size);
+  void UpdateCurrentFencePosition();
+  void UpdateGPUPosition();
+
+  // Waits for as many fences as needed to allocate num_bytes bytes from the buffer.
+  bool WaitForClearSpace(u32 num_bytes);
+
+  u32 m_size = 0;
+  u32 m_current_offset = 0;
+  u32 m_current_space = 0;
+  u32 m_current_gpu_position = 0;
+
+  id<MTLBuffer> m_buffer = nil;
+  u8* m_host_pointer = nullptr;
+
+  // List of fences and the corresponding positions in the buffer
+  std::deque<std::pair<u64, u32>> m_tracked_fences;
+};