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/epaint/src/textures.rs

@@ -214,7 +214,7 @@ impl TexturesDelta {
         self.set.is_empty() && self.free.is_empty()
     }
 
-    pub fn append(&mut self, mut newer: TexturesDelta) {
+    pub fn append(&mut self, mut newer: Self) {
         self.set.extend(newer.set);
         self.free.append(&mut newer.free);
     }