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/image.rs

@@ -301,7 +301,7 @@ impl FontImage {
     }
 
     /// Clone a sub-region as a new image.
-    pub fn region(&self, [x, y]: [usize; 2], [w, h]: [usize; 2]) -> FontImage {
+    pub fn region(&self, [x, y]: [usize; 2], [w, h]: [usize; 2]) -> Self {
         assert!(x + w <= self.width());
         assert!(y + h <= self.height());
 
@@ -311,7 +311,7 @@ impl FontImage {
             pixels.extend(&self.pixels[offset..(offset + w)]);
         }
         assert_eq!(pixels.len(), w * h);
-        FontImage {
+        Self {
             size: [w, h],
             pixels,
         }