[css-grid] grid-auto-flow packing

In the telcon today (which I unfortunately missed), it was decided to
go with sparse packing over dense packing.

Webkit/Blink currently implement dense packing.  (When Julien was
implementing it, it's how he read the algorithm.)  I'm fine with also
offering sparse packing, but if so, I'd like to do it as an option at
*this* level, not the next.

~TJ