Virtual Filesystem (#597)

* Add VfsFile and VfsDirectory classes

* Finish abstract Vfs classes

* Implement RealVfsFile (computer fs backend)

* Finish RealVfsFile and RealVfsDirectory

* Finished OffsetVfsFile

* More changes

* Fix import paths

* Major refactor

* Remove double const

* Use experimental/filesystem or filesystem depending on compiler

* Port partition_filesystem

* More changes

* More Overhaul

* FSP_SRV fixes

* Fixes and testing

* Try to get filesystem to compile

* Filesystem on linux

* Remove std::filesystem and document/test

* Compile fixes

* Missing include

* Bug fixes

* Fixes

* Rename v_file and v_dir

* clang-format fix

* Rename NGLOG_* to LOG_*

* Most review changes

* Fix TODO

* Guess 'main' to be Directory by filename

src/core/file_sys/vfs_offset.h

@@ -0,0 +1,46 @@
+// Copyright 2018 yuzu emulator team
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include "core/file_sys/vfs.h"
+
+namespace FileSys {
+
+// An implementation of VfsFile that wraps around another VfsFile at a certain offset.
+// Similar to seeking to an offset.
+// If the file is writable, operations that would write past the end of the offset file will expand
+// the size of this wrapper.
+struct OffsetVfsFile : public VfsFile {
+    OffsetVfsFile(std::shared_ptr<VfsFile> file, size_t size, size_t offset = 0,
+                  const std::string& new_name = "");
+
+    std::string GetName() const override;
+    size_t GetSize() const override;
+    bool Resize(size_t new_size) override;
+    std::shared_ptr<VfsDirectory> GetContainingDirectory() const override;
+    bool IsWritable() const override;
+    bool IsReadable() const override;
+    size_t Read(u8* data, size_t length, size_t offset) const override;
+    size_t Write(const u8* data, size_t length, size_t offset) override;
+    boost::optional<u8> ReadByte(size_t offset) const override;
+    std::vector<u8> ReadBytes(size_t size, size_t offset) const override;
+    std::vector<u8> ReadAllBytes() const override;
+    bool WriteByte(u8 data, size_t offset) override;
+    size_t WriteBytes(std::vector<u8> data, size_t offset) override;
+
+    bool Rename(const std::string& name) override;
+
+    size_t GetOffset() const;
+
+private:
+    size_t TrimToFit(size_t r_size, size_t r_offset) const;
+
+    std::shared_ptr<VfsFile> file;
+    size_t offset;
+    size_t size;
+    std::string name;
+};
+
+} // namespace FileSys