Consider whether dashed producers should be allowed to flow into other dashed producers

[PathogenDavid]

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

Gonçalo mentioned that the graph built in #1769 should've potentially been impossible to even create in the first place as having a VisualizerMapping⋯⋯VisualizerMapping isn't the intended way to create data mashups.

However it is possible to make something that works outside of the graph presented in that issue. For instance: DashedToDashed.zip

In contrast, this is the "expected" way users should create this data mashup: PointOnImageIssue.zip

(Note the timer in both workflows is a workaround for #1789, ensure it's enabled if you actually run these.)

We should A) consider whether this should be allowed for VisualizerMapping and B) investigate whether other similar invalid-but-sometimes-working situations exist with other dashed producer nodes.