janitor: Fix clippy errors

These are reported as errors by clippy (nothing major IMHO), but they
stop clippy from looking at all files, so let's get them out of the way.

tests/screenshots/testing.rs

@@ -1,6 +1,8 @@
 // Copyright © SixtyFPS GmbH <info@slint-ui.com>
 // SPDX-License-Identifier: GPL-3.0-only OR LicenseRef-Slint-commercial
 
+// cSpell: ignore powf
+
 use std::rc::Rc;
 
 use crossterm::style::Stylize;
@@ -100,7 +102,7 @@ fn compare_images(
 ) -> Result<(), String> {
     let compare = || {
         let reference = image_buffer(reference_path)
-            .map_err(|image_err| format!("error loading reference image: {:#}", image_err))?;
+            .map_err(|image_err| format!("error loading reference image: {image_err:#}"))?;
 
         if reference.size() != screenshot.size() {
             return Err(format!(
@@ -126,7 +128,7 @@ fn compare_images(
                         );
                     }
                 }
-                eprintln!("");
+                eprintln!();
             }
 
             let (failed_pixel_count, max_color_difference) =
@@ -166,18 +168,14 @@ fn compare_images(
 pub fn assert_with_render(path: &str, window: std::rc::Rc<MinimalSoftwareWindow<0>>) {
     let rendering = screenshot(window);
     if let Err(reason) = compare_images(path, &rendering) {
-        assert!(false, "Image comparison failure for {}: {}", path, reason);
+        panic!("Image comparison failure for {path}: {reason}");
     }
 }
 
 pub fn assert_with_render_by_line(path: &str, window: std::rc::Rc<MinimalSoftwareWindow<0>>) {
     let rendering = screenshot_render_by_line(window);
     if let Err(reason) = compare_images(path, &rendering) {
-        assert!(
-            false,
-            "Image comparison failure for line-by-line rendering for {}: {}",
-            path, reason
-        );
+        panic!("Image comparison failure for line-by-line rendering for {path}: {reason}");
     }
 }
 
@@ -188,7 +186,7 @@ pub fn screenshot_render_by_line(
     let width = size.width;
     let height = size.height;
 
-    let mut buffer = SharedPixelBuffer::<Rgb8Pixel>::new(width as u32, height as u32);
+    let mut buffer = SharedPixelBuffer::<Rgb8Pixel>::new(width, height);
 
     // render to buffer
     window.request_redraw();