Data mashups shouldn't require all sources to update

[PathogenDavid]

This is something that came up when Gonçalo and I were discussing #1769

Given the following workflow, a user might reasonably expect the visualizer on the TableLayoutPanel to show the camera capture with the point overlaid on it: PointOnImageIssue.zip

Unfortunately the visualizer will show nothing but a white screen, as it requires both the CreatePoint2f and the CameraCapture to produce a value before it will refresh.

You can see this by enabling the Timer node, which causes the point to flow periodically rather than once.

This isn't necessarily intuitive, so we should consider improving things here. (Gonçalo mentioned this is possibly some optimization logic meant to avoid cloning the image gone wrong.)