Searching for `convert` or PIL has non-negligible performance overhead, so it's
preferable to only do it when really necessary. This way, the search is only
performed when ArtResizer.shared is accessed for the first time.
The previous method was to change self.__class__ dynamically to make __init__
instantiate different classes. This new way, which uses bare functions instead
of separate functor-like classes, instead just forwards the resize() call to
a module-global implementation based on self.method.
Additionally, the semantics of ArtResizer have changed. Clients now *always*
call resize() and proxy_url(), regardless of method. The method makes *one* of
these a no-op. This way, clients need not manually inspect which method is
being used.
artresizer.py instances an ArtResizer object that uses internally the PIL; ImageMagick
or a web proxy service to perform the resizing operations.
Because embedart works on input images located on filesystem it requires PIL or ImageMagick, whereas
fetchart is able to do the job with the fallback webproxy resizer.
This is the first of several commits that will modernize the beets codebase for
Python 2.6 conventions. (Compatibility with Python 2.5 is hereby abandoned.)
The interface no longer specifies the type of the image embedded in the file; it
just returns a bytestring blob. When a type must be stored, it is inferred using
the imghdr module, which shoudl reduce the potential for weird bugs when the
formats don't correspond.
