Add media (WindowGLContext) module in canvas_trait

This module adds a structure (WindowGLContext) which holds the
OpenGL parameters that are going to be used by servo-media player
to render video frames using OpenGL.

In order to fill this structure, three new methods were added to
WindowMethods trait. In this patch only the Glutin-based
implementation provides a simple boilerplate.

The WindowGLContext is created in the entry point of libservo, when
the application window is created, and later passed to the
constellation, the pipeline and to the window element in dom, thus
htmlmediaelement has a mean to obtain these parameters via its
window.

components/canvas_traits/lib.rs

@@ -14,6 +14,7 @@ extern crate malloc_size_of_derive;
 extern crate serde;
 
 pub mod canvas;
+pub mod media;
 #[macro_use]
 pub mod webgl;
 mod webgl_channel;