Implement new WebGL interfaces and methods

This commit implements:
* WebGLFramebuffer
* WebGLRenderbuffer
* WebGLTexture

And adds the following methods to `WebGLRenderingContext`:
* create{Texture,Framebuffer,Renderbuffer}
* bind{Texture,Framebuffer,Renderbuffer}
* destroy{Buffer,Texture,Framebuffer,Renderbuffer}

Fixes:
* WebGLUniform location shouldn't inherit from WebGLObject.

Known Issues:
* WebGL objects have to be destroyed on drop, we may want to keep a reference to the context, or maybe a clone of the renderer to achieve this

Also refactors a huge part of the current implementation, to allow
failing on creation of different WebGL objects.

Blocked on https://github.com/servo/gleam/pull/22

A reftest for most of the added functionality is not doable right now,
we need a few more functions in order to upload a texture, for example.

components/script/dom/webglrenderbuffer.rs

@@ -0,0 +1,39 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// https://www.khronos.org/registry/webgl/specs/latest/1.0/webgl.idl
+use dom::bindings::codegen::Bindings::WebGLRenderbufferBinding;
+use dom::bindings::global::GlobalRef;
+use dom::bindings::js::{Temporary, JSRef};
+use dom::bindings::utils::reflect_dom_object;
+use dom::webglobject::WebGLObject;
+
+#[dom_struct]
+pub struct WebGLRenderbuffer {
+    webgl_object: WebGLObject,
+    id: u32,
+}
+
+impl WebGLRenderbuffer {
+    fn new_inherited(id: u32) -> WebGLRenderbuffer {
+        WebGLRenderbuffer {
+            webgl_object: WebGLObject::new_inherited(),
+            id: id,
+        }
+    }
+
+    pub fn new(global: GlobalRef, id: u32) -> Temporary<WebGLRenderbuffer> {
+        reflect_dom_object(box WebGLRenderbuffer::new_inherited(id), global, WebGLRenderbufferBinding::Wrap)
+    }
+}
+
+pub trait WebGLRenderbufferHelpers {
+    fn get_id(&self) -> u32;
+}
+
+impl<'a> WebGLRenderbufferHelpers for JSRef<'a, WebGLRenderbuffer> {
+    fn get_id(&self) -> u32 {
+        self.id
+    }
+}