Show an OSD message when running a disc image with a large block size

This is intended to catch WIA files which have been created using
wit's default parameters (40 MiB block size), once the WIA PR is
merged. The check does however also work for GCZ files – not that
I think anyone has a GCZ file with a block size that large.

Source/Core/DiscIO/Volume.h

@@ -20,6 +20,7 @@
 
 namespace DiscIO
 {
+class BlobReader;
 enum class BlobType;
 class FileSystem;
 class VolumeWAD;
@@ -132,6 +133,7 @@ public:
   virtual bool IsSizeAccurate() const = 0;
   // Size on disc (compressed size)
   virtual u64 GetRawSize() const = 0;
+  virtual const BlobReader& GetBlobReader() const = 0;
 
 protected:
   template <u32 N>