Atomically rename saved image to avoid race condition with other processes

[0xb8]

I use some other program to monitor output directory and display new images automatically. However, sometimes it opens newly added image too soon, before it's completely written. Because of that the image is not displayed correctly – the program might even crash or hang.

To avoid the race condition, this PR changes the way the image is saved:

1. Write the image with .tmp extension
2. Update EXIF metadata if necessary
3. Atomically rename the file with final extension