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/script_traits/lib.rs

@@ -21,6 +21,7 @@ pub mod webdriver_msg;
 
 use crate::webdriver_msg::{LoadStatus, WebDriverScriptCommand};
 use bluetooth_traits::BluetoothRequest;
+use canvas_traits::media::WindowGLContext;
 use canvas_traits::webgl::WebGLPipeline;
 use crossbeam_channel::{Receiver, RecvTimeoutError, Sender};
 use devtools_traits::{DevtoolScriptControlMsg, ScriptToDevtoolsControlMsg, WorkerId};
@@ -636,6 +637,8 @@ pub struct InitialScriptState {
     pub webrender_api_sender: RenderApiSender,
     /// Flag to indicate if the layout thread is busy handling a request.
     pub layout_is_busy: Arc<AtomicBool>,
+    /// Application window's GL Context for Media player
+    pub player_context: WindowGLContext,
 }
 
 /// This trait allows creating a `ScriptThread` without depending on the `script`