screenshot: Slow to open and display screenshot ui

[ids1024]

As in pop-os/cosmic-workspaces-epoch#59, it seems like displaying the background is introducing some delay. (As well as a bit of delay from something else.)

A solution like pop-os/cosmic-comp#609 works nicely, for window capture; but breaks the way output capture is currently handled in the portal.

Using subsurfaces like cosmic-workspaces might provide some benefit too.