Implement WebXR Raw Camera Access for the ARKit backend

[julapy]

What

Implements the WebXR Raw Camera Access module for the ARKit backend, so Babylon.js content can sample the live AR camera image as a texture inside an immersive-ar session:

const cameraAccess = xr.baseExperience.featuresManager.enableFeature(BABYLON.WebXRFeatureName.RAW_CAMERA_ACCESS, "latest");
cameraAccess.onTexturesUpdatedObservable.add((textures) => {
    material.emissiveTexture = textures[0]; // live ARKit camera image
});

Babylon.js's existing WebXRRawCameraAccess feature works against this unmodified — this PR only adds the native hooks it calls (XRView.camera, XRWebGLBinding.getCameraImage).

How

Commit 1 — xr dependency (ARKit backend):

• Adds CameraTexturePointer / CameraTextureSize to System::Session::Frame::View (null on backends that don't populate them).
• ARKit already converts the camera image YUV→RGB each frame, but only pre-composites it into the color texture — which is the scene render target and can't double as a sampled camera image. This adds a dedicated BGRA8 texture (recreated on viewport size change, released through the same deleted-texture callback as the color/depth textures) and a second camera pass into it, reusing the shared shader via screenPipelineState with the babylonTexture slot left unbound so the shader takes the camera branch.

Commit 2 — NativeXr plugin (JS surface):

• XRCamera (new): exposes width/height and wraps the platform camera texture for bgfx using the same createTexture2D + overrideInternal-on-render-thread retry pattern as NativeCamera's CameraDevice.
• XRView.camera: returns the per-view XRCamera (undefined when the backend provides no camera texture).
• XRWebGLBinding.getCameraImage(camera): returns the camera image as a Napi::Pointer<Graphics::Texture> — the object type NativeEngine uses as a hardware texture's underlyingResource — so WebXRRawCameraAccess can wrap and sample it directly.

The camera texture is viewport-sized and display-aspect-mapped (same image as the passthrough background), which keeps view.projectionMatrix-based intrinsics consistent with what Babylon's _updateCameraIntrinsics computes.

Testing

Verified on device (iPhone 15 Pro Max, iOS 26.5, Xcode 26, Babylon.js 9.9.1 UMD) via the iOS Playground with ar = true and the feature enabled: the feature attaches, onTexturesUpdatedObservable fires with a 1290×2796 texture, and the live camera image renders on scene geometry inside the AR session at a sustained 60 FPS (camera pass adds well under 1 ms GPU time).

Note: testing immersive-ar on current master first requires a fix for #1767 (NativeXr crashes on session start due to a null active encoder after the threading rework); I applied the guard suggested there while validating this PR. This PR is independent of that fix.

Non-ARKit backends are unaffected (CameraTexturePointer stays null → XRView.camera is undefined, which is the same behavior as before this PR).

🤖 Generated with Claude Code

[julapy]

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