feat(MediaViewer): enable GraphicsOffload for videos

In order to allow pass-through of video content to Wayland compositors,
reducing copies on the GPU and thus improving battery life and
performance.

This change should generally be safe to do, however given the higher
chances of triggering driver bugs good testing is due.

src/Views/MediaViewer.vala

@@ -826,6 +826,9 @@ public class Tuba.Views.MediaViewer : Gtk.Widget, Gtk.Buildable, Adw.Swipeable {
 
 		if (media_type.is_video ()) {
 			var video = new Gtk.Video ();
+
+			video.graphics_offload = Gtk.GraphicsOffloadEnabled.ENABLED;
+
 			if (media_type == Tuba.Attachment.MediaType.GIFV) {
 				video.loop = true;
 				video.autoplay = true;