This avoids leaving projects silently vulnerable when this option is set
to a string instead of a one-item tuple containing that string, a very
common misconfiguration.
Since RequestContext doesn't know its Engine until it's passed to
Template.render() -- and cannot without breaking a widely used public
API -- an elaborate hack is required to apply context processors.
Passed the engine instance to loaders. This is a prerequisite for
looking up configuration on the engine instance instead of global
settings.
This is backwards incompatible for custom template loaders that override
__init__. However the documentation doesn't talk about __init__ and the
way to pass arguments to custom template loaders isn't specified. I'm
considering it a private API.
Aymeric Augustin