This is increasingly not just related to screencopy, so it's weird to
add there. I don't see any other module that fits, so add one called
"quirks" (like the Linux kernel uses for device-specific handling in
generic drives).
This allows `cosmic-workspaces` to rely on cosmic-comp for rendering the
background, and just have transparency. This should be a more reliable
and performant way of doing things, at least for now.
Instead of adding another opaque bool argument, this defines an
`ElementFilter` enum, which makes calls more readable.
Window surfaces are still included in screencopy, as needed for the
workspace previews.
