[FIX] Create dummy SceneEntity for unused SceneMeshes

[xeolabs]

This PR changes the way SceneModel handles SceneMesh instances that are not used by any SceneEntity instances.

We cannot have SceneMesh instances that are not used by SceneEntities, because that throws certain renderer counters out of whack.

If we create meshes with SceneModel.createMesh()but don't actually use those meshes with SceneModel.createEntity(), then we end up with internal counters (numVisible, numHighlighted etc) becoming incorrect.

This is because certain internal counts are initialised by SceneModel.createMesh(), independently of whether SceneModel.createEntity() is ever called.

We fixed this in #1336 by making SceneModel buffer the parameters for each SceneModel.createMesh(), without immediately creating the SceneMesh, then created the SceneMesh in SceneModel.createEntity(), only if that SceneMesh was used by the SceneEntity.

This created an undesirable memory spike in SceneModel.finalize() for large models.

We therefore undo that fix (#1336) with this PR and make a better fix that does not create a memory spike.

After this PR

This PR restores SceneModel to its original behaviour of making SceneModel.createMesh() create the SceneMesh immediately, but this time SceneModel.finalize() will create a dummy SceneEntity that owns all unused SceneMesh instances, and logs a warning.

The following example illustrates this.

We create a SceneModel with two extra SceneMeshes, which are copies of one of the yellow table leg and the purple table top, each offset downloads on the Y-axis to separate them visually. A warning is logged in the console, and a dummy SceneEntity is created in the SceneModel to claim them.

import {Viewer, SceneModel} from "../../dist/xeokit-sdk.es.js";

    const viewer = new Viewer({
        canvasId: "myCanvas",
        transparent: true,
        dtxEnabled: true
    });

    viewer.scene.camera.eye = [-21.80, 4.01, 6.56];
    viewer.scene.camera.look = [0, -5.75, 0];
    viewer.scene.camera.up = [0.37, 0.91, -0.11];

    const sceneModel = new SceneModel(viewer.scene, {
        id: "table",
        position: [0, 0, 0],
        scale: [1, 1, 1],
        rotation: [0, 0, 0],
        edges: true,
        idModel: true
    });

    sceneModel.createGeometry({
        id: "myBoxGeometry",
        primitive: "triangles",
        positions: [
            1, 1, 1, -1, 1, 1, -1, -1, 1, 1, -1, 1, 1, 1, 1, 1, -1, 1, 1, -1, -1, 1, 1, -1, 1, 1, 1, 1, 1, -1, -1, 1,
            -1, -1, 1, 1, -1, 1, 1, -1, 1, -1, -1, -1, -1, -1, -1, 1, -1, -1, -1, 1, -1, -1, 1, -1, 1, -1, -1, 1, 1,
            -1, -1, -1, -1, -1, -1, 1, -1, 1, 1, -1
        ],
        indices: [
            0, 1, 2, 0, 2, 3,            // front
            4, 5, 6, 4, 6, 7,            // right
            8, 9, 10, 8, 10, 11,         // top
            12, 13, 14, 12, 14, 15,      // left
            16, 17, 18, 16, 18, 19,      // bottom
            20, 21, 22, 20, 22, 23
        ]
    });

    sceneModel.createMesh({
        id: "redLegMesh",
        geometryId: "myBoxGeometry",
        position: [-4, -6, -4],
        scale: [1, 3, 1],
        rotation: [0, 0, 0],
        color: [1, 0.3, 0.3]
    });

    sceneModel.createEntity({
        id: "redLeg",
        meshIds: ["redLegMesh"],
        isObject: true
    });

    sceneModel.createMesh({
        id: "greenLegMesh",
        geometryId: "myBoxGeometry",
        position: [4, -6, -4],
        scale: [1, 3, 1],
        rotation: [0, 0, 0],
        color: [0.3, 1.0, 0.3]
    });

    sceneModel.createEntity({
        id: "greenLeg",
        meshIds: ["greenLegMesh"],
        isObject: true
    });

    sceneModel.createMesh({
        id: "blueLegMesh",
        geometryId: "myBoxGeometry",
        position: [4, -6, 4],
        scale: [1, 3, 1],
        rotation: [0, 0, 0],
        color: [0.3, 0.3, 1.0]
    });

    sceneModel.createEntity({
        id: "blueLeg",
        meshIds: ["blueLegMesh"],
        isObject: true
    });

    sceneModel.createMesh({
        id: "yellowLegMesh",
        geometryId: "myBoxGeometry",
        position: [-4, -6, 4],
        scale: [1, 3, 1],
        rotation: [0, 0, 0],
        color: [1.0, 1.0, 0.0]
    });

    sceneModel.createMesh({
        id: "yellowLegMeshExcess",
        geometryId: "myBoxGeometry",
        position: [-4, -16, 4],
        scale: [1, 3, 1],
        rotation: [0, 0, 0],
        color: [1.0, 1.0, 0.0]
    });

    sceneModel.createEntity({
        id: "yellowLeg",
        meshIds: ["yellowLegMesh"],
        isObject: true
    });

    sceneModel.createMesh({
        id: "purpleTableTopMesh",
        geometryId: "myBoxGeometry",
        position: [0, -3, 0],
        scale: [6, 0.5, 6],
        rotation: [0, 0, 0],
        color: [1.0, 0.3, 1.0]
    });

    sceneModel.createMesh({
        id: "purpleTableTopMeshExcess",
        geometryId: "myBoxGeometry",
        position: [0, -13, 0],
        scale: [6, 0.5, 6],
        rotation: [0, 0, 0],
        color: [1.0, 0.3, 1.0]
    });

    sceneModel.createEntity({
        id: "purpleTableTop",
        meshIds: ["purpleTableTopMesh"],
        isObject: true
    });

    sceneModel.finalize();