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/VolumeGC.h

@@ -54,6 +54,7 @@ public:
   u64 GetSize() const override;
   bool IsSizeAccurate() const override;
   u64 GetRawSize() const override;
+  const BlobReader& GetBlobReader() const;
 
 private:
   static const u32 GC_BANNER_WIDTH = 96;