per-image immediate-mode variable handling

tweaked prompt generator to process on a per-image basis, allowing 'immediate' `(!__foo__)` variables to be "locked in" per-image, instead of being **either** re-processed on every mention, **or** locked for the entire batch. Note that this does change the behaviour - and no longer allows "locking" variables for an entire batch. (The removed functionality should probably be re-implemented with a new trigger character to differentiate it, or via a configuration option) Note that this makes needing to modify the base library redundant. ( https://github.com/adieyal/dynamicprompts/issues/96 ) It should also be the solution to https://github.com/adieyal/sd-dynamic-prompts/issues/642 and https://github.com/adieyal/sd-dynamic-prompts/issues/717 I believe.