[6.x] Asset modal with checkerboard pattern

[JayGeorge]

This closes #13436 by adding the checkerboard to the asset modal on hover.

A few considerations:

• I believe the checkerboard was designed to not be present so that you can see a clean view of the image
• This can cause problems with certain images, as reported in [6.x] Checkerboard pattern for png's #13436
• Therefore, the checkerboard now shows on hover, similar to the thumbnail. This is likely how you'll see it when you first open the modal, since the hover target is so large—I think this works well. It's more like: if you move your mouse away, you see a clean image without a background.

FWIW, I tried playing with hover transitions here, but they don't work well when the background is initially transparent, like it is here, because the checkerboard effect is layered. I think this is fine—it can just be utilitarian, without a transition.

[JayGeorge]

I changed the approach here show a transparency control instead, only when images support transparency.
The hover approach felt janky on such a big hover target area.

On reflection, I think not showing the checkerboard should be the default. It's pretty rare that you don't simply want to see a clean image. Again—I hope this is the best of both worlds.