Add GLSL Example

Add missing GLSL example

// in shader
```glsl
uniform vec4 a_vec4 = vec4(1, 2, 3, 4);
uniform vec4 b_vec4 = a * 2.0;
 ```

// in JavaScript at init time
```javascript
var someVec2Loc = gl.getUniformLocation(someProgram, "b_vec4");
```