Auto merge of #6380 - ecoal95:webgl-refactoring, r=jdm

Refactor WebGL 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 probably want to keep track of them from the context before).

Some concerns:

**Trait method repetition**:
I'm aware that the traits `WebGL{Buffer,Renderbuffer,Framebuffer,Texture}Helpers` are basically the same, but `delete()` and `id()` methods are everywhere. I've thought something like:

```rust
pub trait WebGLIdentifiable {
    type WebGLId; // id is sometimes i32 (see WebGLUniformLocation)
    fn id(&self) -> Self::WebGLId;
}

pub trait WebGLBindable {
    fn bind(&self);
}

pub trait WebGLDeletable {
    fn delete(&self);
}
```

But I'd want to know your opinion first.

**`renderer` repetition**:
Thought of moving the field: `renderer: Sender<CanvasMsg>` to `WebGLObject`, but I think it makes it way more complicated to read, and also a bit unnecessary, at least IMO (`WebGLObject` will never interact with the field directly). It would also mean that all `WebGLObject`s should have one, which is true at this moment, but maybe not with WebGL 2, for example.

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