Add proper meshes & materials for game objects

The asteroid circles have been replaced with polylines. I was going to
do a regular triangle mesh, but I don't want to figure out GLTF
loading or manually chunking the points into convex hulls.

Asteroid materials are now all populated and their GameAsset getters are
indexed correctly. I'm not sold on the size based color selection.
Later, I think I'll either remove the extra colors or let each of them
randomly apply to any asteroid.

The bullet is now a short line segment. Apparently I already wired in
the rotation logic, so pointing it correctly out of the ship already
works.

src/objects.rs

@@ -83,9 +83,9 @@ pub fn spawn_asteroid(
         };
 
         let collider_radius = match spawn.size {
-            AsteroidSize::Small => 10.0,
-            AsteroidSize::Medium => 20.0,
-            AsteroidSize::Large => 40.0,
+            AsteroidSize::Small => 5.0,
+            AsteroidSize::Medium => 10.0,
+            AsteroidSize::Large => 20.0,
         };
 
         commands.spawn((