Add display model

Sources/SwiftUIMath/Internal/Display/DisplayGlyphRun.swift

@@ -0,0 +1,28 @@
+import CoreGraphics
+import Foundation
+
+extension Math {
+  final class DisplayGlyphRun: DisplayShiftedNode {
+    var glyphs: [UInt16]
+    var offsets: [CGFloat]
+    var font: Math.Font
+
+    init(glyphs: [UInt16], offsets: [CGFloat], font: Math.Font) {
+      self.glyphs = glyphs
+      self.offsets = offsets
+      self.font = font
+      super.init()
+      self.position = .zero
+    }
+
+    override var ascent: CGFloat {
+      get { super.ascent - shiftDown }
+      set { super.ascent = newValue }
+    }
+
+    override var descent: CGFloat {
+      get { super.descent + shiftDown }
+      set { super.descent = newValue }
+    }
+  }
+}