Fix compatibility issues with legacy Catmull sliders

[smoogipoo]

Fixes #18308

[bdach]

Appears to work, but I'll leave one (rather obvious) observation: resolving the issue in this manner makes saving catmull sliders in lazer editor a potentially lossy operation (and one that is lossy in a way that is currently not communicated to the user), i.e. if I modify and save any of the maps mentioned in the issue, the catmull sliders will regress to their broken linear form. I'm not expecting this to be addressed here myself, leaving more as food for thought.

[peppy]

resolving the issue in this manner makes saving catmull sliders in lazer editor a potentially lossy operation

I touched on this when proposing this solution in voice. The way I see it, all curves that are multi-segment on lazer are affected by this. The eventual plan would be to either better communicate this limitation, or implement limited support for the new curve types in stable (for gameplay, not editing) to allow for backwards compatibility.

The second is more likely, and when this is done, stable would use a new pathway that correctly handled multi-segment catmull implicitly. Which is another reason I wanted to keep this isolated to the legacy converter.