Allows using many engines together with sorl-thumbnail

Need to change the THUMBNAIL_ENGINE to sorl_multiengine.engine.pil.Engine

Then, define SORL_ENGINES in your settings with a list of the engines to apply in order:

SORL_ENGINES = [
    'sorl_watermarker.engines.pil.Engine',
	'sorl_sepia.engine.Engine',
]