Negative f for pow() in shaders - debuShader parameter

- Fixes the following errors, because of negative values in pow() function of shaders:
```
(71,64-116): warning X3571: pow(f, e) will not work for negative f, use abs(f) 
or conditionally handle negative values if you expect them
```
-  Optional parameter (debugShaders) for displaying basic warning/errors of shaders.
By default, it is 'true'.

- Example:
```
renderer = new THREE.WebGLRenderer( { antialias: true, debugShaders: false } );
```