R1:324470e39b0d

Give embedded image files with transparency a checkered background

Summary:
Have a consistent rendering experience between File page and file embedded in a task for PNG image files with an alpha channel:
Show a checkered background on the embedded image instead of a white background; show full contrast when hovering over the embedded image.

Closes T15473

Test Plan:
See steps in T15473 - compare an image file embedded in a task before and after applying this patch.

- test in Task description
- test in…