fix three missed conversions to closures.  switch to extending math prototypes rather than replacing them.  This is to ensure that types created in closures within a type's prototype definition get their prototype updated with the full definition.

Renamed Vector3 projectPoint() to applyProjection() as per @WestLangley suggestion in bb873d16.

Changed back some .projectPoint() to .applyMatrix4() as per bb873d16.

Renamed ShaderLib's 'normal2' shader to 'normalmap' as suggested in 4276c9f5.

@WestLangley I've changed most of the vector.applyMatrix4() to vector.projectPoint().
Feel free to change back the ones that aren't needed (or let me know which ones can need to be changed back and I will do it).

ShaderUtils relied on ShaderChunks so this created troubles when trying to use three-canvas and three-extras. Besides, better to keep a single library of shaders.

Plane: Removed normalize() from setFromNormalAndCoplanarPoint and added it back to setFromCoplanarPoints. As suggested in 5c6b5746.

CanvasRenderer+SVGRenderer: Removed 2D clipping for Face3/Face4 elements as the projector is already taking care of that.

However, this can create glitches on extreme cases when the polygon is inside the frustum but the vertices are not.
Maybe this can be achieved with Box3 in a performant way?
/ping @bhouston

This is the implementation of buffergeometry with lines: f53b2753 in webglrenderer2

Added support for object groups in the SceneLoader. "groups" is a new property in the SceneLoader result object.

Ex:
input {
  //...
  "objects" : {
    "Object.1" : {
      "groups"    : [ "Group.A" ],
      //...
    },
    "Object.2" : {
      "groups"    : [ "Group.A", "Group.B" ],
      //...
    },
    "Object.3" : {
      "groups"    : [ "Group.B" ],
      //...
    }
  }
}

result {
  //...
  groups: {
    'Group.A': [ 'Object.1', 'Object.2' ],
    'Group.B': [ 'Object.2', 'Object.3' ]
  },
  //...
}

update math constructors to not copy when provided explicit initial values per @mrdoob: https://github.com/mrdoob/three.js/pull/2902#issuecomment-11999928