VideoCommon: add an asset library that loads directly from the filesystem

Source/Core/VideoCommon/Assets/DirectFilesystemAssetLibrary.h

@@ -0,0 +1,37 @@
+// Copyright 2023 Dolphin Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#pragma once
+#include <chrono>
+#include <filesystem>
+#include <map>
+#include <string>
+
+#include "VideoCommon/Assets/CustomAssetLibrary.h"
+
+namespace VideoCommon
+{
+class CustomTextureData;
+
+// This class implements 'CustomAssetLibrary' and loads any assets
+// directly from the filesystem
+class DirectFilesystemAssetLibrary final : public CustomAssetLibrary
+{
+public:
+  LoadInfo LoadTexture(const AssetID& asset_id, CustomTextureData* data) override;
+
+  // Gets the latest time from amongst all the files in the asset map
+  TimeType GetLastAssetWriteTime(const AssetID& asset_id) const override;
+
+  // Assigns the asset id to a map of files, how this map is read is dependent on the data
+  // For instance, a raw texture would expect the map to have a single entry and load that
+  // file as the asset.  But a model file data might have its data spread across multiple files
+  void SetAssetIDMapData(const AssetID& asset_id,
+                         std::map<std::string, std::filesystem::path> asset_path_map);
+
+private:
+  // Loads additional mip levels into the texture structure until _mip<N> texture is not found
+  bool LoadMips(const std::filesystem::path& asset_path, CustomTextureData* data);
+  std::map<AssetID, std::map<std::string, std::filesystem::path>> m_assetid_to_asset_map_path;
+};
+}  // namespace VideoCommon