Add feature flags to for X11 and Wayland dependencies on Linux

Add 'glx' and 'wayland' feature flags so that we can choose to disable
the dependencies if they are not enabled on gstreamer.

Author: stshine

backends/gstreamer/Cargo.toml

@@ -9,6 +9,10 @@ license = "MPL-2.0"
 name = "servo_media_gstreamer"
 path = "lib.rs"
 
+[features]
+glx = ["servo-media-gstreamer-render-unix/gl-x11"]
+wayland = ["servo-media-gstreamer-render-unix/gl-wayland"]
+
 [dependencies]
 byte-slice-cast = "1"
 glib = { workspace = true }
@@ -38,7 +42,6 @@ url = "2.0"
 [target.'cfg(any(target_os = "linux", target_os = "dragonfly", target_os = "freebsd", target_os = "netbsd", target_os = "openbsd"))'.dependencies]
 servo-media-gstreamer-render-unix = { path = "render-unix", features = [
     "gl-egl",
-    "gl-x11",
 ] }
 
 [target.'cfg(target_os = "android")'.dependencies]