This is a breaking change.
Replaces logic that was previously in our scratch-gui fork.
Upstream does the opposite; they disable cloud variables when video
sensing is used. We can't do the same because of custom extensions.
Disabling all custom extensions when video sensing is used would be too
limiting. The concern with custom extensions is not that they will steal
private skins, but that a project could use their blocks to fetch()
color information to a remote server similar to cloud variables.
The end result is not dissimilar: Cloud variables and video sensing
shouldn't be used together.