Previously we were doing this ad-hoc later in the process but we now
have the `calc` clamping system which can simplify things.
This reveals some false-positives in that we don't handle relative
lengths within these `calc`s but these are fixed in the next commit
Canonicalization can require information that is only known after
compute time (i.e. resolved relative lengths within calcs).
This also allows us to get rid of the `had_explicit_input` flag and just
rely on whether Optional has a value