Add support for unindexed meshes in Collider::from_bevy_mesh

[rlidwka]

fixes #490

implementation was somewhat inspired by assimp/assimp#2049, you can see the explanation of "why" there

Here's how you can test it visually. The glTF file being checked here is this one.

use bevy::prelude::*;
use bevy_rapier3d::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, setup)
        .add_systems(Update, attach_collider)
        .add_plugins(RapierPhysicsPlugin::<NoUserData>::default())
        .add_plugins(RapierDebugRenderPlugin::default())
        .run();
}

fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
    commands.spawn(DirectionalLightBundle::default());
    commands.spawn(Camera3dBundle {
        transform: Transform::from_xyz(0.7, 0.7, 1.0).looking_at(Vec3::new(0.0, 0.3, 0.0), Vec3::Y),
        ..default()
    });
    commands.spawn(SceneBundle {
        scene: asset_server.load("TriangleWithoutIndices.gltf#Scene0"),
        ..default()
    });
}

fn attach_collider(
    mut commands: Commands,
    meshes: Res<Assets<Mesh>>,
    query: Query<(Entity, &Handle<Mesh>), Without<Collider>>,
) {
    for (entity, mesh) in query.iter() {
        let mesh = meshes.get(mesh).unwrap();
        commands
            .entity(entity)
            .insert(Collider::from_bevy_mesh(mesh, &ComputedColliderShape::TriMesh).unwrap());
    }
}