Bevy 0.17 introduces this idea of "Messages" which mostly replace the
previous usage of "Events." The old events still exist, but are meant
more for targetted actions -- something happening and immediately
triggering a function call on an observer.
When the player gets a game-over, the debris is spawned and then frozen
in place as the physics system gets turned off. I can make sure it never
appears by writing the state change and returning early from the
function.
This means the asteroid despawning needs to happen before that point,
otherwise the asteroids would be left on screen.
I'm stealing the thruster mesh, which itself is actually the ship's
mesh, to draw the debris particles. I'll come back and make the scatter
a bit better at some point, but for now it's passable.
There is now a `Weapon` component which is just a timer for the gun's
fire rate. It is ticked every frame, clamping at the "ready" state (0
time remaining).
The ship spawns with this thing, and the `input_ship_shoot` system has
been updated to use it.
