Add an additional constructor method to ManualEventReader, which starts at the current frame's events

[alice-i-cecile]

What problem does this solve or what need does it fill?

#5730 introduced a generally helpful error message,

2022-11-13T19:35:54.363800Z WARN bevy_ecs::event: Missed 345 bevy_input::mouse::MouseMotion events. Consider reading from the EventReader more often (generally the best solution) or calling Events::update() less frequently (normally this is called once per frame). This problem is most likely due to run criteria/fixed timesteps or consuming events conditionally. See the Events documentation for more information.

However, when working with ManualEventReader to read recent events via Events:;get_reader, this error is triggered constantly.

What solution would you like?

The correct solution here, is to instead initialize the manual event reader at the start of the events that occured this frame. This avoids double-reading, and silences the warning.

In order to do this however, bevy_ecs needs to expose a constructor ala get_current that sets the internal events_seen field correctly.

What alternative(s) have you considered?

We could instead (or additionally) allow public construction of EventReader from Events, but this feels more likely to be confused and misused.

Adding public access to ManualEventReader of EventReader may work to solve my particular issue as well, albeit in a bit messier of a way.

[alice-i-cecile]

Note that even if #6596 is merged, I think this is still worth doing: it's a sensible and helpful pattern.