`AbsoluteAxisSolver::solve()` would compute, among other things, the
position of the absolute positioned element if it had start alignment.
Then, `AbsoluteAxisSolver::origin_for_alignment_or_justification()`
could optionally opt into modifying that alignment if needed.
This was quite convoluted and not easy to follow. It's simpler to not
compute the position in `AbsoluteAxisSolver::solve()`, and instead do it
always in `AbsoluteAxisSolver::origin_for_alignment_or_justification()`,
which I'm renaming to `AbsoluteAxisSolver::origin_for_margin_box()`
because it aligns the margin box of the abspos within its alignment
container.
Then the `Anchor` struct becomes useless and can be removed.
Signed-off-by: Oriol Brufau <obrufau@igalia.com>
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