adjust click points to always keep the canvas at the centre of the screen, even when the canvas width is different from the canvas height

fcore/click-points.sml

@@ -12,10 +12,18 @@ struct
 
       val pixelSize = Real32.min (xPixelSize, yPixelSize)
 
+      val actualWidth = pixelSize * realCanvasWidth
+      val actualHeight = pixelSize * realCanvasHeight
+
+      val heightDifference = realWindowHeight - actualHeight
+      val yOffset = heightDifference / 2.0
+      val widthDifference = realWindowWidth - actualWidth
+      val xOffset = widthDifference / 2.0
+
       val xClickPoints = Vector.tabulate (canvasWidth + 1, fn i =>
-        Real32.fromInt i * pixelSize)
+        (Real32.fromInt i * pixelSize) + xOffset)
       val yClickPoints = Vector.tabulate (canvasHeight + 1, fn i =>
-        Real32.fromInt i * pixelSize)
+        (Real32.fromInt i * pixelSize) + yOffset)
     in
       (xClickPoints, yClickPoints)
     end