DiscIO/DirectoryBlob: Add a content source that reads data from a DiscIO::Volume.

Source/Core/DiscIO/DirectoryBlob.cpp

@@ -123,6 +123,15 @@ bool DiscContent::Read(u64* offset, u64* length, u8** buffer) const
         return false;
       }
     }
+    else if (std::holds_alternative<ContentVolume>(m_content_source))
+    {
+      const auto& source = std::get<ContentVolume>(m_content_source);
+      if (!source.m_volume->Read(source.m_offset + offset_in_content, bytes_to_read, *buffer,
+                                 source.m_partition))
+      {
+        return false;
+      }
+    }
     else
     {
       PanicAlertFmt("DirectoryBlob: Invalid content source in DiscContent.");

Source/Core/DiscIO/DirectoryBlob.h

@@ -16,6 +16,7 @@
 #include "Common/CommonTypes.h"
 #include "Common/FileUtil.h"
 #include "DiscIO/Blob.h"
+#include "DiscIO/Volume.h"
 #include "DiscIO/WiiEncryptionCache.h"
 
 namespace File
@@ -57,9 +58,23 @@ struct ContentPartition
   u64 m_partition_data_offset;
 };
 
-using ContentSource = std::variant<ContentFile,      // File
-                                   const u8*,        // Memory
-                                   ContentPartition  // Partition
+// Content chunk that loads data from a Volume.
+struct ContentVolume
+{
+  // Offset from the start of the volume for the first byte represented by this chunk.
+  u64 m_offset;
+
+  // The volume to read data from.
+  const Volume* m_volume;
+
+  // The partition passed to the Volume's Read() method.
+  Partition m_partition;
+};
+
+using ContentSource = std::variant<ContentFile,       // File
+                                   const u8*,         // Memory
+                                   ContentPartition,  // Partition
+                                   ContentVolume      // Volume
                                    >;
 
 class DiscContent