[css3-grid-layout] Handling of orthogonal writing-mode on the grid items

Hi everyone,

the specification is silent about how to handle orthogonal flow on the
grid items e.g.:

<div style="display: grid; grid-definition-rows: 100px;
grid-definition-columns: 100px;">
   <div style="writing-mode: vertical-rl;"></div>
   <div></div>
</div>

The intuition would be that that the measure of each grid items would
be 100px. Their extent would be 0 as they have no content.

The sequential (columns then rows) algorithm in the specification
cannot handle this example as you need access to both track directions
when laying all children (due to the width: auto).

The question starts to get even more interesting when you add
content-based sizing to the grid element.

Thanks,
Julien