Fix cached images not appearing in ImageWithFallback

[Bouh]

Fix a race condition because of the cache.
Images loaded from browser cache don't trigger the onLoad event, causing opacity to remain at 0 and images to be invisible.

GDevelop/newIDE/app/src/UI/ImageWithFallback.js

Lines 83 to 98 in bfb60b4

	<CorsAwareImage
	{...props}
	src={src}
	alt={alt}
	// Keep the image invisible until it has successfully loaded, so the
	// browser's native "broken image" glyph is never shown for a fraction of
	// a second before the fallback appears.
	style={{
	...style,
	opacity: isLoaded ? (style && style.opacity) || 1 : 0,
	}}
	loading={loading}
	onLoad={() => setIsLoaded(true)}
	onError={() => setHasError(true)}
	/>
	);

Solution

Check the img.complete property to detect cached images and set isLoaded state.
Works for both fresh loads and cached images.