Fixed a number of issues with the changes to fetchart:
- Remove redundant fetches. This was making the Amazon source download every
image twice even when art resizing was not enabled!
- Restore local_only switch in plugin hook, which got lost in the shuffle at
some point.
- Don't replace the original image file in-place; use a temporary file instead.
This would clobber the original source image on the filesystem with the
downscaled version!
This allows matches to indicate both missing and unmatched tracks in their
candidates and solves some of the spaghetti tuples that were passed around
during autotagging.
