Add display model

Sources/SwiftUIMath/Internal/Display/DisplayGlyph.swift

@@ -0,0 +1,27 @@
+import CoreGraphics
+import Foundation
+
+extension Math {
+  final class DisplayGlyph: DisplayShiftedNode {
+    var glyph: UInt16
+    var font: Math.Font
+
+    init(glyph: UInt16, font: Math.Font, range: NSRange) {
+      self.glyph = glyph
+      self.font = font
+      super.init()
+      self.position = .zero
+      self.range = range
+    }
+
+    override var ascent: CGFloat {
+      get { super.ascent - shiftDown }
+      set { super.ascent = newValue }
+    }
+
+    override var descent: CGFloat {
+      get { super.descent + shiftDown }
+      set { super.descent = newValue }
+    }
+  }
+}