FR: Add a time check to is-facing sexp

[Durandal-Thoth]

Pretty straight forward, this is a request to add an optional argument for a length of time (probably in ms) for a ship to be facing another ship for it to return true.

[Goober5000]

This is not really practical. Generally speaking, sexps do not have "memory" like this. It would be necessary to essentially duplicate a sexp in code form, adding extra infrastructure for this special case.

Instead, this can be achieved using sexps within the mission itself. Set up a series of events so that a timestamp is assigned when the is-facing condition is initially satisfied, and cleared when the condition is not satisfied. Then add another event to check whether the timestamp is larger than your desired duration.

[MjnMixael]

This is actually a good and useful idea and many sexps could benefit from this sort of thing. Generally speaking we should be making FREDing easier and more accessible by constantly lowering the bar of "programming think" required and having a framework in place to do this for various sexps as it comes up seems good and useful to expand our sexp system.

I'm not able to work this for the FSO side until I get BtA done at the very least, but it's something I'd be interested in creating a system for at some point.

Copying The_E's idea from Discord for safekeeping.

if target not facing object: reset timestamp for object and return false if target facing object and timestamp not set: set timestamp for object if target facing object and timestamp set: if timestamp is old enough: return true else return false

[Goober5000]

I neglected to update this ticket back in November, but this request has been implemented as a scripted SEXP:

facing-sct.tbm.txt facing-sexp.tbm.txt

This pattern can be applied to other similar use cases that require time checks.

[Goober5000]

A eureka moment made this request a whole lot more tractable, not just for is-facing but for any boolean operator or SEXP tree. And the infrastructure needed was minimal and non-invasive. Amazing how much of a difference the right design can make.

This has now been implemented via #6415.