Performance improvements

- Use larger buffer size when writing uncompressed files
  (was libcurl's default of 16 kb, change it to 128 kb)
- Uncompress the next MB of data, while it is hashing/writing to
  disk in seperate thread.

imagewriter.cpp

@@ -172,6 +172,7 @@ void ImageWriter::startWrite()
     else
     {
         _thread = new DownloadThread(urlstr, _dst.toLatin1(), _expectedHash, this);
+        _thread->setInputBufferSize(IMAGEWRITER_UNCOMPRESSED_BLOCKSIZE);
     }
 
     _powersave.applyBlock(tr("Downloading and writing image"));