Rewrite host GPU abstraction

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

src/util/opengl_stream_buffer.h

@@ -0,0 +1,53 @@
+// SPDX-FileCopyrightText: 2019-2023 Connor McLaughlin <stenzek@gmail.com>
+// SPDX-License-Identifier: (GPL-3.0 OR CC-BY-NC-ND-4.0)
+
+#pragma once
+
+#include "opengl_loader.h"
+
+#include "common/types.h"
+
+#include <memory>
+#include <string_view>
+#include <tuple>
+#include <vector>
+
+class OpenGLStreamBuffer
+{
+public:
+  virtual ~OpenGLStreamBuffer();
+
+  ALWAYS_INLINE GLuint GetGLBufferId() const { return m_buffer_id; }
+  ALWAYS_INLINE GLenum GetGLTarget() const { return m_target; }
+  ALWAYS_INLINE u32 GetSize() const { return m_size; }
+
+  void Bind();
+  void Unbind();
+
+  void SetDebugName(const std::string_view& name);
+
+  struct MappingResult
+  {
+    void* pointer;
+    u32 buffer_offset;
+    u32 index_aligned; // offset / alignment, suitable for base vertex
+    u32 space_aligned; // remaining space / alignment
+  };
+
+  virtual MappingResult Map(u32 alignment, u32 min_size) = 0;
+
+  /// Returns the position in the buffer *before* the start of used_size.
+  virtual u32 Unmap(u32 used_size) = 0;
+
+  /// Returns the minimum granularity of blocks which sync objects will be created around.
+  virtual u32 GetChunkSize() const = 0;
+
+  static std::unique_ptr<OpenGLStreamBuffer> Create(GLenum target, u32 size);
+
+protected:
+  OpenGLStreamBuffer(GLenum target, GLuint buffer_id, u32 size);
+
+  GLenum m_target;
+  GLuint m_buffer_id;
+  u32 m_size;
+};