Merge data textures and VBOs into same SceneModel class

[xeolabs]

Changes in this PR

• Unify vertex buffer object (VBO) and data texture (DTX) modes into the same SceneModel class
• SceneModel is then no longer an abstract type - becomes a concrete class
• Update loader plugins to instantiate SceneModel
• VBOSceneModel and PerformanceModel alias SceneModel for backwards compatibility
• Create set of examples to separately test each SceneModel model definition and rendering mode

Also:

• Make LoD culling work for data textures and VBOs
• Make View frustum culling (VFC) work for DTXs and VBOs
• Make snap-to-vertex/edge work for DTX and VBOs
• SceneModel will automatically select whether to use VBOs or DTX internally, for each contained object.
• SceneModel will use VBOs for some objects, and DTX for others, as required.
• It will always attempt to use DTX by default for objects that don't have textures.
• SceneModel will use only use DTX for objects that have triangles, and always use VBOs for objects with point or line primitives.

However for testing, and when we want to force data textures, we can force SceneModel to use data textures for all objects that have triangles, like so:

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

Terms

• VBO = Vertex Buffer Objects
• DTX = Data Textures
• VFC = View Frustum Culling
• LoD - Level-of-Detail culling

Just a heads-up @tmarti and @Kurtil - please hold off on any big PRs to master branch for the next few days. This PR makes some big refactorings of your recent PRs, unifying them into the same SceneModel class. Small PRs should be OK though, if they're small enough to enable me to manually copy them into this branch.

[ghost]

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