Swipe mode issues on transparency maps

[aparlato]

Maps that have transparency less than 1 all the way through will show the second map when in swipe mode. This is because in swipe mode the maps are overlaid on top of one another and use clip paths to swipe between them.

We should add a blank or checkered background below each map. This will require that we remove this solid background when highlight diff mode is active because of how the CSS works to allow that since it also relies on overlaid maps.