Added Matrix3 uniforms loading

Added handling of mat3 uniforms (both single and arrays) to loadUniformsGeneric() in WebGLRenderer.

The Matrix3 syntax follows the convention of the [current Matrix4 loading syntax](https://github.com/mrdoob/three.js/wiki/Uniforms-types). In the uniforms object:

| Type Property Value | Uniform |
| ------------- | ------------- |
| "m3"  | single Matrix3  |
| "m3v"  | Matrix3 array  |