Fix table track offsets when there is visibility: collapse
Each non-collapsed track used to increase the offset by the subsequent
border spacing. Now they will take care of their preceding spacing
instead.
This way, if a cell spans two rows, and the second is collapsed, the
cell won't be forced to be at least as tall as the border spacing.
This matches Gecko and Blink (WebKit lacks visibility: collapse).
This makes visibility-collapse-border-spacing-001.html fail because we
generate outlines in a different way than Blink. Gecko also fails it
in a similar (but different) way.