add function to convert from absolute pixel value to NDC

temp-squares/fcore/ndc.sml

@@ -24,7 +24,7 @@ struct
 
   fun fromPixelX (xpos, windowWidth, windowHeight) =
     let
-      val halfWidth = (Real32.fromInt windowWidth) / 2.0
+      val halfWidth = Real32.fromInt (windowWidth div 2)
       val xpos = xpos - halfWidth
     in
       if windowWidth > windowHeight then
@@ -40,7 +40,7 @@ struct
 
   fun fromPixelY (ypos, windowWidth, windowHeight) =
     let
-      val halfHeight = (Real32.fromInt windowHeight) / 2.0
+      val halfHeight = Real32.fromInt (windowHeight div 2)
       val ypos = ~(ypos - halfHeight)
     in
       if windowHeight > windowWidth then