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/mod.rs

@@ -332,6 +332,9 @@ pub mod webglprogram;
 pub mod webglrenderingcontext;
 pub mod webglshader;
 pub mod webgluniformlocation;
+pub mod webgltexture;
+pub mod webglframebuffer;
+pub mod webglrenderbuffer;
 pub mod websocket;
 pub mod window;
 pub mod worker;