Move towards replacing F.foldM with streaming, using mutable accumulator

src/Picture/Data.hs

@@ -22,6 +22,7 @@ data Verx = Verx
     , _vxCol    :: !Point4
     , _vxType   :: !VertexType
     , _vxLayer  :: !Int
+    , _vxShadNum  :: !ShadNum
     }
 data VertexType
     = PolyV
@@ -31,6 +32,16 @@ data VertexType
     | ArcV      !Point3
     | EllV
 
+newtype ShadNum = ShadNum { _unShadNum :: Int }
+
+polyNum, polyzNum, bezNum, textNum, arcNum, ellNum :: ShadNum
+polyNum     = ShadNum 0
+polyzNum    = ShadNum 1
+bezNum      = ShadNum 2
+textNum     = ShadNum 3
+arcNum      = ShadNum 4
+ellNum      = ShadNum 5
+
 type RGBA  = Point4
 type Color = Point4