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/webgluniformlocation.rs

@@ -28,11 +28,11 @@ impl WebGLUniformLocation {
 }
 
 pub trait WebGLUniformLocationHelpers {
-    fn get_id(self) -> i32;
+    fn id(self) -> i32;
 }
 
 impl<'a> WebGLUniformLocationHelpers for &'a WebGLUniformLocation {
-    fn get_id(self) -> i32 {
+    fn id(self) -> i32 {
         self.id
     }
 }