Extrude preview: a New Body has its own colour, apart from Join (#1805) (#1861)

Thanks @DuBearte.

crates/ui-egui/src/preview_drag_tests.rs

@@ -3,7 +3,7 @@ use super::*;
 use serde_json::json;
 
 fn colors() -> Colors {
-    Colors { body: [1, 1, 1, 255], added: [2, 2, 2, 255], removed: [3, 3, 3, 100], edge: [0, 0, 0, 255] }
+    Colors { body: [1, 1, 1, 255], added: [2, 2, 2, 255], new_body: [2, 2, 2, 255], removed: [3, 3, 3, 100], edge: [0, 0, 0, 255] }
 }
 
 fn fixture() -> Session {