WebGLRenderer2: Fixed a possible issue when modifying scene parameters and having only one program (thx alteredq)

Projector::projectScene now does a first pass through Projector::projectObject (which does frustum culling and object sorting).
So now CanvasRenderer and SVGRenderer feature frustum culling too.

The most tangible practical effect is that demos with multiple Lucys / Walts are now initialized much faster ;).

Reuse is achieved mostly by moving mesh chunks / VBOs (formerly called materialFaceGroups) from Mesh into Geometry.

This means sorting of geometry by face materials + breaking large geometries into chunks now has to be done after construction of geometry.

It becomes a step in the usual sequence:

    geometry.computeNormals();
    geometry.computeCentroids();
    geometry.sortFacesByMaterial();

If geometry is constructed programmatically (e.g. with GeometryUtils.merge), this has to be called afterwards.

Also of note: single geometry cannot be reused both for flat and smooth shading, as only one stream of normals is baked into VBO.

MeshUtils.merge( object1, object2 [ geometry | mesh ] ): Merges object2 into object1. Implemented in materials_cubemap_sky.