This adds parallelism using rayon to the initial layout of flex lines
and line item. There is still no support for parallelism when laying out
line items again to account for stretch. This can be done in a followup
change.
In addition, the array of data for each line item is collected into a
new structure, `FlexLineItem`. This prevents a lot of needless zipping
of iterators at the expense of some new vector allocations. A folluwup
change can likely unify this structure and `FlexItemLayoutResult` though
that will require a larger refactor.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>
fe0701e226