Make windows thumbnails use their native stacking order as a 'z' value
for current desktop while lowering windows from other desktops.

By lowering others instead of raising current ones we get to use
stacking order compatible with desktop & dock/panel window types.

For now they are drawn separately, so this change doesn't do much on its
own, but it would become useful during refactoring when putting all
windows (regular and panel types) in a single parent with shared z stack.