UI: Remove individual sources (for now) from vcam config
Individual sources cannot be used as vcam outputs (yet) due to the fact
that they cannot currnetly scale. If a source is not exactly the same
dimensions as the canvas, there will be blackness on the right and/or
bottom of the source. It's just not currently usable as-is.
Scenes are a much better fit for this because users can transform
sources within scenes as they please to fit the canvas.
At a later point, we'll likely reintroduce this feature as long as
they're automatically transformed or something.
jp9000