port improvements to autobrake_test too, excluding the weird calc_targets thing

autobrake_test.p8

@@ -136,12 +136,8 @@ function ship:draw()
 end
 
 function calc_velocity(v0, t, vmax, drag)
- local v1 = v0 + t
- local sg = sgn(v1)
- v1 -= sg*drag
- if (sgn(v1) != sg) return 0
- if (abs(v1) > vmax) return sg*vmax
- return v1
+ v0 = mid(v0 + t, vmax, -vmax)
+ return v0 - mid(drag, -drag, v0)
 end
 
 function ship:update()
@@ -191,7 +187,7 @@ function constraints:constrain(s, want)
  if (bx <= txm) return want
  self.color = 8
  local overage = bx - txm
- want -= overage/(bf+1)
+ want -= overage/max(bf,1)
  if (want < -s.thrust) want = -s.thrust
  return want
 end