- the context exists for the lifecycle of one card's render,
and caches calls to things like .card() to avoid add-ons needing to
do their own cache management.
- add-ons can optionally add extra data to the context if they need
it across multiple filters
- removed card_will_render. the legacy hook is still available for
now
- card_did_render is now called only once, with both front and back
text
In the process of factoring out the field filtering, the "extra"
and "fullname" args are just passed in as a blank string now.
Extra was functionality that allowed a field modifier to be defined
as "filtername(arg1,arg2):field", and fullname was the name of the
field including any provided field modifiers. From grepping through
the add-ons on AnkiWeb, neither appears to have been used.
