workspace: Cap scaled floating size to target layout size

A window preview's floating geometry is scaled down according to the workspace's allocation, while the layout geometry is computed directly for the available space.

For previews that maintain their real size in the layout geometry, that scaling leads to a distracting size bounce when transitioning between both layouts.

Address that by not allowing the scaled floating size to drop below that layout size (which is at most equal to the unscaled floating size).

Related: #2968