return tuple from function 'startBuildTextLineGap' as function to move LineGap, used in 'startBuildTextLineGap' optimises nodes for better performance on subsequent operations

fcore/buffer.sml

@@ -205,6 +205,6 @@ buildTextString ( startIdx, rStrHd, [], 5, 5, 5
              * else we can do. *)
             []
     in
-      Vector.concat acc
+      (Vector.concat acc, lineGap)
     end
 end

shell/shell.sml

@@ -23,7 +23,7 @@ struct
       val lineGap = ioToLineGap (io, LineGap.empty)
       val _ = TextIO.closeIn io
 
-      val textVec = Buffer.startBuildTextLineGap (0, lineGap, 1920, 1080)
+      val (textVec, _) = Buffer.startBuildTextLineGap (0, lineGap, 1920, 1080)
       val shellState = GlDraw.create window
       val shellState = GlDraw.uploadText (shellState, textVec)