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/dom/window.rs

@@ -68,6 +68,7 @@ use crate::webdriver_handlers::jsval_to_webdriver;
 use app_units::Au;
 use base64;
 use bluetooth_traits::BluetoothRequest;
+use canvas_traits::media::WindowGLContext;
 use canvas_traits::webgl::WebGLChan;
 use crossbeam_channel::{unbounded, Sender, TryRecvError};
 use cssparser::{Parser, ParserInput, SourceLocation};
@@ -318,6 +319,10 @@ pub struct Window {
     /// Replace unpaired surrogates in DOM strings with U+FFFD.
     /// See <https://github.com/servo/servo/issues/6564>
     replace_surrogates: bool,
+    
+    /// Window's GL context from application
+    #[ignore_malloc_size_of = "defined in script_thread"]
+    player_context: WindowGLContext,
 }
 
 impl Window {
@@ -481,6 +486,10 @@ impl Window {
     pub fn unminify_js(&self) -> bool {
         self.unminify_js
     }
+
+    pub fn get_player_context(&self) -> WindowGLContext {
+        self.player_context.clone()
+    }
 }
 
 // https://html.spec.whatwg.org/multipage/#atob
@@ -2074,6 +2083,7 @@ impl Window {
         is_headless: bool,
         replace_surrogates: bool,
         user_agent: Cow<'static, str>,
+        player_context: WindowGLContext,
     ) -> DomRoot<Self> {
         let layout_rpc: Box<dyn LayoutRPC + Send> = {
             let (rpc_send, rpc_recv) = unbounded();
@@ -2154,6 +2164,7 @@ impl Window {
             unminify_js,
             userscripts_path,
             replace_surrogates,
+            player_context,
         });
 
         unsafe { WindowBinding::Wrap(runtime.cx(), win) }