This implements the Default trait for `AxesOverflow`, to avoid having to
manually set both axes to `Overflow::Visible`.
Also implements the conversion from `&ComputedValues` to `AxesOverflow`,
which is the also used for overflow viewport propagation.
And moves the `is_inline_box()` checkinto the `ignores_overflow` logic.
Testing: Not needed, no change in behavior
Signed-off-by: Oriol Brufau <obrufau@igalia.com>