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).
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.
