Add display model

Sources/SwiftUIMath/Internal/Display/DisplayList.swift

@@ -0,0 +1,44 @@
+import CoreGraphics
+import Foundation
+
+extension Math {
+  final class DisplayList: DisplayNode {
+    enum LinePosition: Int {
+      case regular
+      case `subscript`
+      case superscript
+    }
+
+    var linePosition: LinePosition = .regular
+    var children: [DisplayNode] = []
+    var index: Int = NSNotFound
+
+    init(children: [DisplayNode], range: NSRange) {
+      super.init()
+      self.children = children
+      self.position = .zero
+      self.index = NSNotFound
+      self.range = range
+      recomputeDimensions()
+    }
+
+    func recomputeDimensions() {
+      var maxAscent: CGFloat = 0
+      var maxDescent: CGFloat = 0
+      var maxWidth: CGFloat = 0
+      for node in children {
+        let ascent = max(0, node.position.y + node.ascent)
+        maxAscent = max(maxAscent, ascent)
+
+        let descent = max(0, 0 - (node.position.y - node.descent))
+        maxDescent = max(maxDescent, descent)
+
+        let width = node.width + node.position.x
+        maxWidth = max(maxWidth, width)
+      }
+      self.ascent = maxAscent
+      self.descent = maxDescent
+      self.width = maxWidth
+    }
+  }
+}