Background should move with content when switching workspaces workspaces

• The current animation between workspaces, especially the touchpad one, moves only the windows without actually moving the background, making it rather unrealistic: the user is supposed to see a different view, no matter if the background is the same, not just open windows "flying" on the screen with no "physical" justification.

• This approach is already used on Windows, Mac OS, Elementary, Deepin and Android, and it needs some design on the current implementation of background images in GNOME.

• Adding a solid color border between workspaces in such animations might also make the division between workspaces clearer to the user.