Several bugfixes and performance improvements.

- Set jpeg subsampling to 2, 2 again, as it seems to be the correct value.
- Move opening files to ExifTransfer::copyMetadata, so that no copy is done if that fails.
- Prepare an empty raw buffer for the preview JPEG instead of unpacking a whole image. This may fail in the future...
- Set output params for the preview JPEG before calling LibRaw::open_file.
- Fix changing the edit tool with Ctrl/Shift so that it remembers the last tool in use.
- Change edit radius with the mouse wheel.
- Fix repainting edit brush after changing its radius.
Fix the preview size to its right value.