Add trajectory to grenades, various refactoring

src/Dodge/Default/Shell.hs

@@ -4,15 +4,16 @@ import Dodge.Data
 import Picture
 import Geometry.Data
 
-defaultShell :: Projectile
+defaultShell :: Prop
 defaultShell = Shell
     { _pjPos = V2 0 0
+    , _pjZ   = 20
     , _pjStartPos = V2 0 0
     , _pjVel = V2 0 0
     , _pjAcc = V2 0 0
     , _pjDir = 0
     , _pjSpin = 0
-    , _pjDraw = const blank
+    , _prDraw = const (mempty, blank)
     , _pjID = 0
     , _pjTimer = 0
     , _pjUpdate = const id