bktr: Implement IVFC offset shifting

Fixes base game read errors

src/core/loader/loader.h

@@ -214,6 +214,15 @@ public:
         return true;
     }
 
+    /**
+     * Gets the difference between the start of the IVFC header and the start of level 6 (RomFS)
+     * data. Needed for bktr patching.
+     * @return IVFC offset for romfs.
+     */
+    virtual u64 ReadRomFSIVFCOffset() const {
+        return 0;
+    }
+
     /**
      * Get the title of the application
      * @param title Reference to store the application title into

src/core/loader/nca.cpp

@@ -71,6 +71,12 @@ ResultStatus AppLoader_NCA::ReadRomFS(FileSys::VirtualFile& dir) {
     return ResultStatus::Success;
 }
 
+u64 AppLoader_NCA::ReadRomFSIVFCOffset() const {
+    if (nca == nullptr)
+        return 0;
+    return nca->GetBaseIVFCOffset();
+}
+
 ResultStatus AppLoader_NCA::ReadProgramId(u64& out_program_id) {
     if (nca == nullptr || nca->GetStatus() != ResultStatus::Success)
         return ResultStatus::ErrorNotInitialized;

src/core/loader/nca.h

@@ -37,6 +37,7 @@ public:
     ResultStatus Load(Kernel::SharedPtr<Kernel::Process>& process) override;
 
     ResultStatus ReadRomFS(FileSys::VirtualFile& dir) override;
+    u64 ReadRomFSIVFCOffset() const override;
     ResultStatus ReadProgramId(u64& out_program_id) override;
 
 private: