Support ray intersects mesh instances

[scarletsky]

This is a solution for https://forum.playcanvas.com/t/way-to-get-mesh-instance/4646 .
And the code is migrated from THREE.js 😃

Usage is learned from app.systems.rigidbody.raycastFirst:

var meshInstances = entity.model.model.meshInstances;
app.mouse.on('mousemove', function(e) {
    camera.camera.screenToWorld(e.x, e.y, camera.camera.nearClip, fromPoint);
    camera.camera.screenToWorld(e.x, e.y, camera.camera.farClip, toPoint);
    ray.origin.copy(fromPoint);
    ray.direction.copy(toPoint).sub(ray.origin).normalize();
  
    var results = app.systems.camera.raycastMeshInstances(ray, meshInstances);
    console.log(results);
});

The result:

[
    {
        index: Number, // the index of triangle based on IndexBuffer
        distance: Number, // the distance between ray.origin and the hit point
        point: Vec3, // the hit point of world
        localPoint: Vec3, // the hit point of local
        normal: Vec3, // the triangle's normal
        vertices: [Vec3, Vec3, Vec3], // the triangles's vertices
        meshInstance: meshInstance // the meshInstance of this triangle
    }
]

I've made a demo in https://scarletsky.github.io/engine/, you can open the console and see the performance and the result.

[mvaligursky]

no need to unlock, as we read only from it .. otherwise gpu version is updated

[willeastcott]

@mvaligursky + @slimbuck: This is our oldest PR (nearly 3 years!!). We need to figure out what to do with it.

My personal positive feelings are:

1. It's cool to be able to raycast without having to depend on the physics engine.
2. It's cool that this PR works with skinned mesh instances.
3. Code looks reasonably straightforward and clean.

And negative feelings are:

1. What about batched stuff?
2. What about morphed mesh instances?
3. Is performance good enough?

[scarletsky]

@willeastcott
This PR just scan all triangles of mesh, and its performance may not so good.