Add hatched background to Image widget preview area

[bahia0019]

In the Image widget, if you have a white logo with transparent areas (PNG or SVG), it's impossible to see the logo because the preview background is white.
I'm submitting a patch that creates a hatched area. It's the exact same CSS as the background area for the Media Library edit screen.

[bahia0019]

The pertinent changes start at line 15 with the class ".media-widget-control .media-widget-preview img". It appears Prettier has made some formatting changes to the CSS.

[pikamander2]

I know this should be a separate ticket, but I've seen similar issues happen when uploading transparent favicons, expect with those the background seems to appear black. That might be worth looking into as well at the same time.

[davidbaumwald]

@SergeyBiryukov Is this still doable for 5.4?

[davidbaumwald]

This ticket still needs testing, and with 5.4 Beta 1 landing tomorrow, this is being moved to Future Release. If any maintainer or committer feels this can be included in 5.4 or wishes to assume ownership during a specific cycle, feel free to update the milestone accordingly.

[SergeyBiryukov]

Previously: #33299, #41948, #42535.

[SergeyBiryukov]

In 47257:

Media: Improve the preview of transparent images in Image widget by using CSS3 to show a checkered background.

Props bahia0019.
Fixes #49237.