Auto merge of #6293 - ecoal95:webgl-objects, r=nox

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.

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tests/wpt/mozilla/tests/mozilla/interfaces.html

@@ -180,6 +180,9 @@ var interfaceNamesInGlobalScope = [
   "WebGLRenderingContext",
   "WebGLUniformLocation",
   "WebGLBuffer",
+  "WebGLFramebuffer",
+  "WebGLRenderbuffer",
+  "WebGLTexture",
   "WebGLProgram",
   "WebGLShader",
   "WebGLObject",