Transform: set transformEnded properly and don't dedup tags.

Deduplicating override tags when a transform has ended is dangerous as
it will do bad things in the case of temporally overlapping transforms.

While this is an incredibly small edge case, it's still one to be
avoided.
Transform: make error message more useful.
Transform: take shortcuts with interpolation boundaries.

I'm pretty sure I won't have to revert this later?