Add convenience methods for determining if collisions are starting or stopping

[ndarilek]

Objective

It can be tricky to match and handle all of an entity's collision states in a single event loop. I routinely find myself writing code like this:

    for Collision(contacts)in events.read() {
        let started = contacts.during_current_frame && !contacts.during_previous_frame;
        let stopped = !contacts.during_current_frame && contacts.during_previous_frame;
        if started {
        } else if stopped {
        }

Solution

Add Contacts::collision_started and Contacts::collision_stopped to more conveniently check these conditions.

Changelog

Added

• Added Contacts::collision_started and Contacts::collision_stopped for more conveniently checking if a collision just started or stopped.