Moved AnimationBatch out of Scene, because Scene doesn't ever use it. Instead, PreviewForm now has _animationBatch. The reason AnimationBatch should be moved is because it's not directly tied to the renderer, and even has usage outside of rendering (with exporting). The only ties it has to Scene is resetting and setting up MeshBatch, and that could theoretically be moved out if needed.
Added VRAM.AssignModelTextures in-place of manually assigning textures to models in PreviewForm.
Both AnimationBatch and VRAM can now operate even if scene passed in the constructor is null. This is because they have utility outside of just being used for the renderer.
AnimationBatch now calls Invalidate in its constructor, to ensure the first frame is always processed.