DiscIO: Allow converting from formats other than ISO and GCZ

The constant DESIRED_BUFFER_SIZE was determined by multiplying the
old hardcoded value 32 with the default GCZ block size 16 KiB.
Not sure if it actually is the best value, but it seems fine.

Source/Core/DiscIO/CompressedBlob.h

@@ -52,6 +52,7 @@ public:
   u64 GetRawSize() const override { return m_file_size; }
   u64 GetDataSize() const override { return m_header.data_size; }
   bool IsDataSizeAccurate() const override { return true; }
+  u64 GetBlockSize() const override { return m_header.block_size; }
   u64 GetBlockCompressedSize(u64 block_num) const;
   bool GetBlock(u64 block_num, u8* out_ptr) override;