Optimize network IO #15

Users report Network drives and NAS storage can be slow. Couple of optimizations I can think of:

• Switched from memory-mapped files to normal file IO. This seems faster on networks in my tests.
• Locating any temporary files used during the update process on the same network drive. Diffractor normally creates a temporary updated copy of a photo and then replaces the original with it. Currently, the copy is created in the temp folder – but that is slow if the temp folder is on a different drive to the updated file.