Update euclid.

There are a few canvas2d-related dependencies that haven't updated, but they
only use euclid internally so that's not blocking landing the rest of the
changes.

Given the size of this patch, I think it's useful to get this landed as-is.

components/webrender_traits/lib.rs

@@ -4,7 +4,7 @@
 
 #![deny(unsafe_code)]
 
-use euclid::Size2D;
+use euclid::default::Size2D;
 use std::collections::HashMap;
 use std::sync::{Arc, Mutex};
 use webrender_api::units::TexelRect;