add TextScroll function for centering line to cursor, and use it in 'NormalFinish.centreToCursor' function

fcore/normal-mode/normal-finish.sml

@@ -128,7 +128,6 @@ struct
         )
     end
 
-  (* todo: check; the way we get the startLine is almost certainly wrong *)
   fun centreToCursor (app: app_type) =
     let
       val
@@ -146,8 +145,7 @@ struct
 
       val buffer = LineGap.goToIdx (cursorIdx, buffer)
       val cursorLine = LineGap.getLineNumberOfIdx (cursorIdx, buffer)
-      val startLine =
+      val startLine = TextScroll.getLineCentre (cursorLine, windowHeight)
-        TextScroll.getStartLine (prevLineNumber, cursorLine, windowHeight div 2)
 
       val buffer = LineGap.goToLine (startLine, buffer)
       val drawMsg = NormalModeTextBuilder.build

fcore/text-scroll.sml

@@ -34,7 +34,6 @@ struct
       (* cursorLine > prevLineNumber *)
       let
         val howManyLinesWeCanFit = windowHeight div TC.ySpace
-        val howManyLinesWeCanFit = howManyLinesWeCanFit
       in
         if cursorLine > prevLineNumber + (howManyLinesWeCanFit - 3) then
           (* cursorLine is after the visible part of the screen
@@ -43,4 +42,12 @@ struct
         else
           prevLineNumber
       end
+
+  fun getLineCentre (cursorLine, windowHeight) =
+    let
+      val howManyLinesWeCanFit = windowHeight div TC.ySpace
+      val startLine = cursorLine - (howManyLinesWeCanFit div 2)
+    in
+      Int.max (startLine, 0)
+    end
 end