Add ability to use values passed to grid-template-columns and
grid-template-rows for CSS Grid layout within a repeat() function.
E.g. grid-template-columns: repeat(2, 50px); means to have two columns
of 50px width each.
When the indicated column-span is greater than the implicit grid (like
in cases when the grid has the default size of 1x1, and the column is
supposed to span any number greater than that), then previously were
crashing.
Ensure that when a grid item is passed with a span and a fixed end
position, that if the resulting start of this item is less than 0 then
it won't throw. This is a temporary measure until the correct
functionality is implemented.
Implement span correctly when indicated in the grid-column-start,
grid-row-start, etc. CSS properties. Previously it had been implemented
as if span was something that went alongside the position property, but
actually it seems like if you do 'span 3' in the grid-column-start
property, for example, this means it literally spans 3 blocks, and the
3 has nothing to do with position.
