compositor: Do not parse the Cargo.lock file while building (#33222)

The compositor's `build.rs` script was parsing the `Cargo.lock` file in
order to tag WebRender captures with the WebRender version. The embedder
already knows what version of Servo we are using, which should be enough
to infer the WebRender revision. This changes does that and generally
does a bit of cleaning up of how captures are done.

- The name of the capture directory is now `webrender-captures`
- There is console output now when captures are done. Before it was hard
  to know if it succeeded.
- Simplify the Compositor constructor a little to avoid passing
  arguments so much.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/compositing/windowing.rs

@@ -224,6 +224,11 @@ pub trait EmbedderMethods {
         None
     }
 
+    /// Returns the version string of this embedder.
+    fn get_version_string(&self) -> Option<String> {
+        None
+    }
+
     /// Returns the protocol handlers implemented by that embedder.
     /// They will be merged with the default internal ones.
     fn get_protocol_handlers(&self) -> ProtocolRegistry {