Betterboxing: make the decorator border disappear when the corners are flat

Reported by @thorin, who provided also the screenshot: the decorator line from #42387 (closed), which should usually blend with the letterboxing background, is unpleasantly visible when the window is at letterboxing-rounded size.

Since the decorator is useful only when the corners are rounded, and we auto-flatten them whenever the letterboxing margin is less than the rounded corners radius, we can hide the decorator as soon as the corner are flattened and bring it back when they're rounded again.

While we're here, per @donuts 's request, we'll remove the thin shadow separating the toolbar from the browser content, originally implemented in !886 (comment 2984761).