Run a glib loop for the GStreamer backend

This is a workaround for an issue where GStreamer does not deliver end
of stream signals unless there is a main loop running. See [1] for
more information.

1. #393 (comment)

backends/gstreamer/Cargo.toml

@@ -25,6 +25,7 @@ ipc-channel = { workspace = true }
 lazy_static = "1.2.0"
 log = "0.4"
 mime = "0.3.13"
+once_cell = "1.18.0"
 servo-media = { path = "../../servo-media" }
 servo-media-audio = { path = "../../audio" }
 servo-media-gstreamer-render = { path = "render" }

backends/gstreamer/lib.rs

@@ -50,6 +50,7 @@ use gst::ClockExt;
 use ipc_channel::ipc::IpcSender;
 use media_stream::GStreamerMediaStream;
 use mime::Mime;
+use once_cell::sync::OnceCell;
 use registry_scanner::GSTREAMER_REGISTRY_SCANNER;
 use servo_media::{Backend, BackendInit, SupportsMediaType};
 use servo_media_audio::context::{AudioContext, AudioContextOptions};
@@ -78,6 +79,8 @@ lazy_static! {
     static ref BACKEND_BASE_TIME: gst::ClockTime = gst::SystemClock::obtain().get_time();
 }
 
+static BACKEND_THREAD: OnceCell<bool> = OnceCell::new();
+
 pub struct GStreamerBackend {
     capture_mocking: AtomicBool,
     instances: Arc<Mutex<HashMap<ClientContextId, Vec<(usize, Weak<Mutex<dyn MediaInstance>>)>>>>,
@@ -94,6 +97,12 @@ impl GStreamerBackend {
         plugin_dir: PathBuf,
         plugins: &[&'static str],
     ) -> Result<Box<dyn Backend>, ErrorLoadingPlugins> {
+
+        BACKEND_THREAD.get_or_init(|| {
+            thread::spawn(|| glib::MainLoop::new(None, false).run());
+            true
+        });
+
         gst::init().unwrap();
 
         let mut errors = vec![];