Allow picking viewer screenshots instead of render when uploading a new version

[mruegenberg]

Currently when I click the "Take screenshot" button in the "Add preview revisions to publish", it tries to get a render. There should be an option to get a viewport screenshot as well, which dccutils already supports.

In the same vein, the corresponding button for animation tries to take a "render animation", which is not very practical as animators would usually want to create a playblast instead.

[frankrousseau]

ping @EvanBldy

[PeteDraper]

seconded (and 40th'd for the rest of the team..!)

[PeteDraper]

(pls also include camera BG...!)