Created a separate variable for cc normal, independent perturb implementations

This fixes rendering of screen space effects depending on normals in the presence of a global clipping plane.

* Implemented user-defined clipping planes.

* Renderer: Minor documentation update.

* Implements object-level clipping.

* Renderer: Minor cosmetic changes.

* Fixed lethal typo in normal shader.

* Let shadows respect .side of object's material.

* Fixed Plane.applyMatrix4.

- Scale denormalized but did not scale the position
- Projection is not supported (solved by documentation)
- Removed unnecessary copying to make up for the normalization
- Still arguably too heavy

* Added custom clipping example.

* Clipping: Fixed transition to disabled state.

* Improved custom clipping example.

- GUI dependency of volume visualization on Enabled state
- Larger radius for cylindrical global clipping area

* No .matrixAutoUpdate = false in clipping example.

* Let ShadowMap check renderer.localClippingEnabled.

* Added a simplified clipping example.

* trying to unify packing.  Untested.

* adopt packing.glsl in ShaderSkin.js

* adopt fract because theoretically it should be faster, no divide.

* remove whitespace.

* Revert "adopt fract because theoretically it should be faster, no divide."

This reverts commit 8dfb3f1d2a53d389540bb514d5fc183605e9b133.