webgl: Refactor implementation to move logic inside the DOM interfaces

This improves the encapsulation and consistency in our WebGL
implementation.

Also allows to implement new methods such as `getShaderSource()`.

It will also allow us to use `delete()` in the destructors of them (note
that we will want to keep track of them from the context).

components/script/dom/webidls/WebGLRenderingContext.webidl

@@ -569,7 +569,7 @@ interface WebGLRenderingContextBase
     //WebGLShaderPrecisionFormat? getShaderPrecisionFormat(GLenum shadertype, GLenum precisiontype);
     DOMString? getShaderInfoLog(WebGLShader? shader);
 
-    //DOMString? getShaderSource(WebGLShader? shader);
+    DOMString? getShaderSource(WebGLShader? shader);
 
     //any getTexParameter(GLenum target, GLenum pname);