servo/main: Load gl symbols in headless mode

This allows running WebGL reftests in wpt.

components/servo/Cargo.toml

@@ -133,6 +133,9 @@ features = ["plugins"]
 [dependencies.gleam]
 version = "0.1"
 
+[dependencies.offscreen_gl_context]
+git = "https://github.com/ecoal95/rust-offscreen-rendering-context"
+
 [dependencies]
 env_logger = "0.3"
 time = "0.1.12"

components/servo/main.rs

@@ -29,12 +29,25 @@ extern crate log;
 extern crate servo;
 extern crate time;
 
+extern crate gleam;
+extern crate offscreen_gl_context;
+
+use gleam::gl;
+use offscreen_gl_context::GLContext;
 use servo::Browser;
 use servo::compositing::windowing::WindowEvent;
 use servo::net_traits::hosts;
 use servo::util::opts;
 use std::rc::Rc;
 
+#[cfg(not(target_os = "android"))]
+fn load_gl_when_headless() {
+    gl::load_with(|addr| GLContext::get_proc_address(addr) as *const _);
+}
+
+#[cfg(target_os = "android")]
+fn load_gl_when_headless() {}
+
 fn main() {
     env_logger::init().unwrap();
 
@@ -47,6 +60,9 @@ fn main() {
     hosts::global_init();
 
     let window = if opts::get().headless {
+        // Load gl functions even when in headless mode,
+        // to avoid crashing with webgl
+        load_gl_when_headless();
         None
     } else {
         Some(app::create_window(std::ptr::null_mut()))