Require ContentSize on UiImage again (#16138)

# Objective

The `ContentSize` requirement on `UiImage` got lost during merge
conflict fixes, causing some images such as the icons on the `game_menu`
example to disappear.

Fixes #16136

## Solution

Require `ContentSize` on `UiImage` again.

---------

Co-authored-by: Alice Cecile <[email protected]>

crates/bevy_ui/src/widget/image.rs

@@ -12,7 +12,7 @@ use taffy::{MaybeMath, MaybeResolve};
 /// The 2D texture displayed for this UI node
 #[derive(Component, Clone, Debug, Reflect)]
 #[reflect(Component, Default, Debug)]
-#[require(Node, UiImageSize)]
+#[require(Node, UiImageSize, ContentSize)]
 pub struct UiImage {
     /// The tint color used to draw the image.
     ///