* Text: merge variable font axes by tag and cap their number
Each tag in a variations object scanned the style's growing axis list, so a
large object cost quadratic time, twice (validation probe and apply). The
object is now merged in one keyed pass, refused when it names more than 64
axes, and a merge that would leave a style with more than 64 is refused
without changing the style.
* cargo fmt
* address review: refuse variation over every run before changing any
* address review: empty-document range cannot overflow in set_attr
Dylan M. Taylor
e1ea71f540