Use Self everywhere (#3787)

This turns on the clippy lint
[`clippy::use_self`](https://rust-lang.github.io/rust-clippy/v0.0.212/index.html#use_self)
and fixes it everywhere.

crates/emath/src/rect_transform.rs

@@ -34,7 +34,7 @@ impl RectTransform {
         self.to.size() / self.from.size()
     }
 
-    pub fn inverse(&self) -> RectTransform {
+    pub fn inverse(&self) -> Self {
         Self::from_to(self.to, self.from)
     }