Perform deco calculation for plan

This uses a bunch of default values that we eventually need to get from
the UI, but it's a first step towards a working dive planner.

This exhibits some graphical artifacts when running, but other than that
appears to be mostly correct.

Things go far worse if I enable the changing of the scale once the deco
makes the dive longer than the displayed time window. Things quickly
spiral out of control.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>