#36 0% bridgeheads can support a bridge

[Geoff Cashman]

A 100% bridgehead that is redesignated into something
else will immediately cause any connecting bridgespans
to collapse if they are redesignated to some other sector
type. This is perhaps not bad in itself, as it allows a
player to blow a bridge quickly.

However, if that same bridgespan it connected to
another bridgehead sector that is at 0%, the bridgespan
will not drop.

This is silly. 60% is often used as a criteria for
functionality of various sector types. A bridgehead's
ability to support a bridgespan should be tied to that
60% measure as well.

[Markus Armbruster]

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Bridgespans and -towers collapse when damaged below 20%.
Bridgespans need a supporting -head or tower, which doesn't
need to be efficient. If we require it to be 60%, it's
easier to splash a bridge by attacking the supports than the
bridgespan. Unintuitive. If we should decide to require
eff, then we should require 20%.

By the way, I hate designate immediately splashing bridges.
It's a death trap for the player: extremely costly typos.
It's also unintuitive, because `designate' usually just
schedules a change for the next update.

[Markus Armbruster]

Fixed in commit 4824648, v4.3.33. Required efficiency is 20%.