Windows thumbnail generation bug

[lehitoskin]

Need to ensure the temporary thumbnail is placed inside a path to which we have write access. That is, the thumbnails directory. Possibly a good idea to name it the encoded md5_ivy-tmp.png instead of just the image name.

open-output-file: error deleting file
  path: C:\Program Files (x86)\Ivy Image Viewer\1528169817_ivy-tmp.png
  system error: The system cannot find the file specified.; errno=2
  context...:
   c:\users\lehi\documents\racket\ivy\thumbnails.rkt:45:2: for-loop

[IonoclastBrigham]

Are we pretty sure this isn't an issue for POSISH systems?

[lehitoskin]

Oops! Looks like this issue was already fixed at some point because running from latest Master generates no errors.