surive at 0hp and adjust hp values to match

Instead of doing a special case for 1HP, 0HP is survivable, ships die
at negative HP instead. all ship health is adjusted to match, assuming
the weakest shot is 0.5hp, which is currently true. the "real" game will
totally rebalance all ships and weapons anyway.

we're getting close to when I have to stop dawdling and implement the
real game, the engine is _there._

vacuum_gambit.p8

@@ -339,15 +339,15 @@ function drawhud()
  inset(120,66,125,125)
  -- 57 px vertically
  local mxs, cs, mxh, ch = primary_ship.maxshield, primary_ship.shield, primary_ship.maxhp, primary_ship.hp
- if (mxs > 0) and (mxh > 1) then
+ if (mxs > 0) and (mxh > 0) then
-  local split = 57 * (mxs / (mxs + mxh-1)) \ 1 + 66
+  local split = 57 * (mxs / (mxs + mxh)) \ 1 + 66
   line(121, split, 124, split, 0xba)
   vertmeter(121,67,124,split-1,cs, mxs,shlcols)
-  vertmeter(121,split+1,124,124,ch-1, mxh-1, hpcols)
+  vertmeter(121,split+1,124,124,ch, mxh, hpcols)
  elseif mxs > 0 then
   vertmeter(121,67,124,124,cs,mxs,shlcols)
- elseif mxh > 1 then
+ elseif mxh > 0 then
-  vertmeter(121,67,124,124,ch-1,mxh-1,hpcols)
+  vertmeter(121,67,124,124,ch,mxh,hpcols)
  else
   print("!", 122, 94, 9)
   print("!", 121, 93, 8)
@@ -441,7 +441,7 @@ mknew(ship_m)
 
 function ship_m:die()
  self.dead = true
- if (self.hp <= 0) boom(self.x+self.size*4, self.y+self.size*4,12*self.size, self.boss)
+ if (self.hp < 0) boom(self.x+self.size*4, self.y+self.size*4,12*self.size, self.boss)
 end
 
 function ship_m:calc_velocity(v0, t)
@@ -551,7 +551,7 @@ function ship_m:hitsomething(dmg)
  dmg -= self.shield
  self.shield = 0
  self.hp -= dmg
- if self.hp <= 0 then
+ if self.hp < 0 then
   self:die()
   return true
  end
@@ -916,11 +916,10 @@ player = ship_m.new{
 mknew(player,
  function(p)
   p.main_gun = zap_gun.new()
-  -- ONE HIT MODE: max hp is
+  -- ONE HIT MODE
-  -- minimum nonzero value.
   --
-  -- p.hp = 0x0.0001
+  -- p.hp = 0
-  -- p.maxhp = 0x0.0001
+  -- p.maxhp = 0
   -- p.shield = 0
   -- p.maxshield = 0
  end
@@ -940,7 +939,7 @@ frownie = ship_m.new{
  sparkodds = 8,
  
  -- health and power
- hp = 1, -- enemy ships need no max hp
+ hp = 0.5, -- enemy ships need no max hp
 
  -- position
  x=60, -- x and y are for upper left corner
@@ -962,7 +961,7 @@ mknew(frownie)
 
 blocky = frownie.new{
  sprite = 10,
- hp = 2,
+ hp = 1.5,
  hurt = {
   x_off = 0,
   y_off = 0,
@@ -971,7 +970,7 @@ blocky = frownie.new{
  },
  
  ow = function(self)
-  if self.hp <= 1 then
+  if self.hp < 1 then
    self.sprite = 11
   else
    self.sprite = 10
@@ -990,7 +989,7 @@ spewy = frownie.new{
   width=8,
   height=5
  },
- hp=1,
+ hp=0.5,
  maxpower=70,
  generator=0.5,
  fire_off_x=4,
@@ -1015,7 +1014,7 @@ chasey = ship_m.new{
  },
  sparks = smokespark,
  sparkodds = 8,
- hp = 2,
+ hp = 1.5,
  shield = 1,
  maxshield = 1,
  shieldcost = 180,
@@ -1047,7 +1046,7 @@ xl_chasey=chasey.new{
   width = 12,
   height = 10
  },
- hp = 20,
+ hp = 19.5,
  shield = 5,
  boss = true,
  slip = false,