BufferGeometry: refactor normalizeNormals()

- added gitter badge to readme per issue #11614

Clipping Intersection Example: Added plane helpers

Added overrideColor parameter to LightHelpers

WebGL clipping example: render shadows double-sided

WebGLShadowMap: Proper shadows when clipIntersection is true

Add map/alphaMap support to distanceRGBA shader

Added PlaneHelper and Box3Helper

Clarified exception warning

Plane docs: Correct some inaccuracies and clean up

Plane.js: Refactoring and clean up

Material.html: Correct clipping plane description

ColladaLoader2: Better skeleton parsing

A clipped mesh is typically not a closed mesh.

ColladaLoader2: Fix parsing of texture parameters

make Matrix(x) error messages correct and consistent

Fix mesh raycast bug when has multi-material

Uncomment usage of TextDecoder in GLTFLoader

The TextDecoder block was added in d681e0f0, but it seems to have been commented out (for debugging purposes, maybe?) in the GLTFLoader.