ShaderBuilder / refactoring to use an actual builder pattern

There is now a ShaderBuilder class with chainable methods
to specify more easily the contents of the final shaders.
This is to avoid passing around large objects to different functions,
and allow for a richer API.

The documentation has also been corrected and clarified.

src/ol/layer/WebGLPoints.js

@@ -3,7 +3,7 @@
  */
 import {assign} from '../obj.js';
 import WebGLPointsLayerRenderer from '../renderer/webgl/PointsLayer.js';
-import {getSymbolFragmentShader, getSymbolVertexShader, parseSymbolStyle} from '../webgl/ShaderBuilder.js';
+import {parseSymbolStyle} from '../webgl/ShaderBuilder.js';
 import Layer from './Layer.js';
 
 
@@ -82,8 +82,8 @@ class WebGLPointsLayer extends Layer {
    */
   createRenderer() {
     return new WebGLPointsLayerRenderer(this, {
-      vertexShader: getSymbolVertexShader(this.parseResult_.params),
-      fragmentShader: getSymbolFragmentShader(this.parseResult_.params),
+      vertexShader: this.parseResult_.builder.getSymbolVertexShader(),
+      fragmentShader: this.parseResult_.builder.getSymbolFragmentShader(),
       uniforms: this.parseResult_.uniforms,
       attributes: this.parseResult_.attributes
     });