Implement ObjectFit::ScaleDown for images (#10063)

While working towards fixes for the image viewer, @mikayla-maki and I
discovered that we didn't have `object-fit: scale-down` implemented.
This doesn't _fully_ solve the image issues as there is some issue where
only the bounds width is updating on layout change that I haven't fully
chased down.

Co-Authored-By: @mikayla-maki 

Release Notes:

- N/A

---------

Co-authored-by: Mikayla Maki <mikayla@zed.dev>

crates/gpui/src/elements/img.rs

@@ -99,6 +99,8 @@ pub enum ObjectFit {
     Contain,
     /// The image will be scaled to cover the bounds of the element.
     Cover,
+    /// The image will be scaled down to fit within the bounds of the element.
+    ScaleDown,
     /// The image will maintain its original size.
     None,
 }
@@ -114,7 +116,7 @@ impl ObjectFit {
         let image_ratio = image_size.width / image_size.height;
         let bounds_ratio = bounds.size.width / bounds.size.height;
 
-        match self {
+        let result_bounds = match self {
             ObjectFit::Fill => bounds,
             ObjectFit::Contain => {
                 let new_size = if bounds_ratio > image_ratio {
@@ -137,6 +139,42 @@ impl ObjectFit {
                     size: new_size,
                 }
             }
+            ObjectFit::ScaleDown => {
+                // Check if the image is larger than the bounds in either dimension.
+                if image_size.width > bounds.size.width || image_size.height > bounds.size.height {
+                    // If the image is larger, use the same logic as Contain to scale it down.
+                    let new_size = if bounds_ratio > image_ratio {
+                        size(
+                            image_size.width * (bounds.size.height / image_size.height),
+                            bounds.size.height,
+                        )
+                    } else {
+                        size(
+                            bounds.size.width,
+                            image_size.height * (bounds.size.width / image_size.width),
+                        )
+                    };
+
+                    Bounds {
+                        origin: point(
+                            bounds.origin.x + (bounds.size.width - new_size.width) / 2.0,
+                            bounds.origin.y + (bounds.size.height - new_size.height) / 2.0,
+                        ),
+                        size: new_size,
+                    }
+                } else {
+                    // If the image is smaller than or equal to the container, display it at its original size,
+                    // centered within the container.
+                    let original_size = size(image_size.width, image_size.height);
+                    Bounds {
+                        origin: point(
+                            bounds.origin.x + (bounds.size.width - original_size.width) / 2.0,
+                            bounds.origin.y + (bounds.size.height - original_size.height) / 2.0,
+                        ),
+                        size: original_size,
+                    }
+                }
+            }
             ObjectFit::Cover => {
                 let new_size = if bounds_ratio > image_ratio {
                     size(
@@ -162,7 +200,9 @@ impl ObjectFit {
                 origin: bounds.origin,
                 size: image_size,
             },
-        }
+        };
+
+        result_bounds
     }
 }