No more headless compositor. Just the normal one.

This changes headless operation to strictly be a runtime option, rather
than a compile-time one. Note that the old headless version still relied
on a display server to support WebGL, while it now requires one all the
time.

Fixes #8573

ports/glutin/Cargo.toml

@@ -7,10 +7,6 @@ authors = ["The Servo Project Developers"]
 name = "glutin_app"
 path = "lib.rs"
 
-[features]
-window = []
-headless = ["servo-glutin/headless"]
-
 [dependencies]
 bitflags = "0.4"
 url = {version = "0.5.7", features = ["heap_size"]}