Simplify line breaking
Simplify line breaking

When we break an inline child twice, we have to add the initial (absolute) skip
stack and the new (relative) skip stack. The old code needed to check that the
children were the same before adding the stacks, but this detection was buggy
and complicated. The new implementation works even if the broken children are
not the same, removing the need to detect the previously broken child.

Fix #923.

Kozea/WeasyPrint,Kozea/WeasyPrint