Staging controller improvements

[AntonKuzin]

First - just skip any other checks if there already is a reason to decouple.
InverseStageHasActiveEngines can sometimes produce the incorrect results, for example, when there's a sustainer stage with boosters - it may decide there are no active engines because the sustainer was activated on the previous stage.

[lamont-granquist]

That seems really weird that would be buggy since every R7 or drop-stage atlas out there should have a sustainer stage with boosters?

Do you have a craft file with a replication case?

[AntonKuzin]

Sure. It's just a regular souyz without a fairing. Make sure to setup a hotstaging(1s is enough) and use a 4x physical timewarp.
I think the lack of fairing is what exposed this bug - with a fairing the function just returns before reaching this code.

[AntonKuzin]

@lamont-granquist any luck with reproducing the bug?

[lamont-granquist]

Yeah I got a repro. That craft worked, and it did seem to require 4x warp. So this seems to be a case where for a tick, at high physwarp, the engines have the old inverseStage even though the currentStage on the Vessel has been decremented, but I wanted to do just a little more digging and ponder the staging control code (generally) with that in mind.