Pull logic for clamping corner rounding radii out of Corners::to_pixels (#27460)

This seems more correct as corners are not necessarily only for rounding
radii.

Also applies clamping after scaling in `paint_quad`, deduplicating that
logic. This also provides a more precise result by doing the clamping
after scaling, avoiding floating point rounding issues (probably a
non-issue).

Release Notes:

- N/A

crates/gpui/src/elements/img.rs

@@ -421,7 +421,7 @@ impl Element for Img {
             window,
             cx,
             |style, window, cx| {
-                let corner_radii = style.corner_radii.to_pixels(bounds.size, window.rem_size());
+                let corner_radii = style.corner_radii.to_pixels(window.rem_size());
 
                 if let Some(Ok(data)) = source.use_data(window, cx) {
                     let new_bounds = self