Add display model

Sources/SwiftUIMath/Internal/Display/DisplayLine.swift

@@ -0,0 +1,25 @@
+import CoreGraphics
+import Foundation
+
+extension Math {
+  final class DisplayLine: DisplayNode {
+    var inner: DisplayList?
+    var lineShiftUp: CGFloat = 0
+    var lineThickness: CGFloat = 0
+
+    init(inner: DisplayList?, position: CGPoint, range: NSRange) {
+      self.inner = inner
+      super.init()
+      self.position = position
+      self.range = range
+    }
+
+    override var position: CGPoint {
+      didSet { updateInnerPosition() }
+    }
+
+    private func updateInnerPosition() {
+      inner?.position = CGPoint(x: position.x, y: position.y)
+    }
+  }
+}