New thumbnail generators can be dropped in and will be made available by
Ansel providing:
1. The class name is as: Ansel_ImageGenerator_{type}Thumb and filename
matches, i.e. {type}Thumb.php where {type} is the unique name for your
thumbnail type.
2. Implements a _create() method that applies the effects to the image
(see existing generators for how this works).
3. If a matching "stack" generator is desired, that should be named
similarly: Ansel_ImageGenerator_{type}ThumbStack with matching filename:
{type}ThumbStack.php
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