Now that the positions of each grid item have been calculated, and the
sizes of the individual rows and columns ascertained, can actually
layout the different items.
According to the spec it's necessary to:
1. Layout the cells in the grid
2. Find the sizes of the rows and columns
Since I had started to do this backwards, and as I expand in future
commits, I take here the opportunity to start with a clean state.
The occupation_grid keeps track of which cells in the grid have been
filled out.
This is a really starter attempt at formatting the grid. It doesn't yet
take into account the computed_values of grid-template-rows, nor the
values in grid-column-start and like CSS properties.
But these changes are a start and make it so the examples in
display-grid.html work.
To be fleshed out further..
