Reflect the video below the hinge in the tabletop layout

The iPhone Duo tabletop layout mirrors every presented frame into the
lower half, blurred and fading out, like a screen standing on a glossy
table. The render view blits a quarter-resolution copy of the display
framebuffer into a ring of IOSurface-backed buffers (no Y flip, so the
surface is already mirrored) and hands it to a CALayer hosted by
MPVReflectionView; the simulator's software renderer feeds the same
layer a flipped CGImage. Off during PiP, for audio-only streams, and
via the new Playback → Foldable → "Reflection Below Video" setting.

Yattee/Services/Player/MPV/MPVReflectionLayer.swift

@@ -0,0 +1,52 @@
+//
+//  MPVReflectionLayer.swift
+//  Yattee
+//
+//  Layer that shows a vertically mirrored copy of the rendered video frame,
+//  used for the "reflection" below the video in the iPhone Duo tabletop
+//  layout. The render view feeds it every presented frame.
+//
+
+import Foundation
+import QuartzCore
+import IOSurface
+
+#if canImport(UIKit)
+import UIKit
+
+final class MPVReflectionLayer: CALayer {
+    override init() {
+        super.init()
+        contentsGravity = .resizeAspect
+        isOpaque = false
+        // Frame updates must not trigger implicit fades.
+        actions = ["contents": NSNull(), "bounds": NSNull(), "position": NSNull(), "transform": NSNull()]
+    }
+
+    override init(layer: Any) {
+        super.init(layer: layer)
+    }
+
+    required init?(coder: NSCoder) {
+        super.init(coder: coder)
+    }
+
+    /// Show a surface whose rows are already mirrored (GL blit without Y flip).
+    func display(_ surface: IOSurface) {
+        contents = surface
+    }
+
+    /// Show an upright image; the layer mirrors it itself.
+    func displayMirrored(_ image: CGImage) {
+        if CATransform3DIsIdentity(transform) {
+            transform = CATransform3DMakeScale(1, -1, 1)
+        }
+        contents = image
+    }
+
+    func clear() {
+        contents = nil
+    }
+}
+
+#endif