In convert_engine, at this point ``source_image.source`` is already deleted (it's a temporary copy). See [convert_engine.py](https://github.com/mariocesar/sorl-thumbnail/blob/master/sorl/thumbnail/engines/convert_engine.py#L63)

Does anyone know, can we get rid of [using temporary images](https://github.com/mariocesar/sorl-thumbnail/blob/master/sorl/thumbnail/engines/convert_engine.py#L69) in convert engine altogether?
This would potentially simplify code and speed up execution. 
My guess, tmp file is used to work around permissions, or are there any other considerations?

Otherwise we need specific logic to keep track of ``source_image.source`` while we create multiple thumbnails from same source.

// cc @relekang @mariocesar @philipn