Add programmatic GPU frame capture via TestUtils.captureNextFrame.

[bkaradzic-microsoft]

Introduces a one-shot, cross-backend API for requesting a GPU debugger capture of the next submitted frame. Wraps bgfx's BGFX_FRAME_DEBUG_CAPTURE flag, so the same call works with Xcode Metal capture on macOS/iOS, PIX or RenderDoc on Windows, and RenderDoc on Linux/Android.

Primary use case: producing a .gputrace for a failing validation test without having to manually time a capture in the IDE. Add "capture": true to a test entry in Apps/Playground/Scripts/config.json and the test harness will request a capture on the screenshot frame; with the platform GPU debugger attached, a capture document opens automatically.

Implementation:

• DeviceImpl owns an std::atomic<bool> m_captureNextFrame. Frame() consumes it via exchange(false) and ORs BGFX_FRAME_DEBUG_CAPTURE into the bgfx::frame() flags, guaranteeing the request is applied to exactly one frame even under concurrent JS-thread calls.
• DeviceContext exposes RequestCaptureNextFrame()as a thin forwarder, extending the existing public surface alongside RequestScreenShot.
• TestUtils plugin adds a cross-platform captureNextFrame() JS method (no per-platform .mm/.cpp variants required).
  validation_native.js honors an optional per-test "capture" flag and arms a capture before the final currentScene.render() of the screenshot frame.

When no GPU debugger is attached the bgfx flag is a no-op, so the API is safe to leave in JS that ships in release builds.

[Copilot]

Pull request overview

Adds a one-shot API to request a GPU debugger capture of the next submitted frame, exposed from the graphics device up through the TestUtils JS plugin and consumed by the Playground validation harness.

Changes:

• Introduces DeviceContext::RequestCaptureNextFrame() and forwards it into DeviceImpl.
• Implements one-shot capture behavior via an atomic flag consumed in DeviceImpl::Frame() to set a bgfx frame capture flag.
• Adds TestUtils.captureNextFrame() and wires validation_native.js to optionally arm capture on the screenshot frame (test.capture).

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
Plugins/TestUtils/Source/TestUtils.h	Exposes captureNextFrame as a JS instance method.
Plugins/TestUtils/Source/TestUtils.cpp	Implements JS method by forwarding to DeviceContext.
Core/Graphics/Source/DeviceImpl.h	Adds atomic state and public request method on DeviceImpl.
Core/Graphics/Source/DeviceImpl.cpp	Implements capture request + applies bgfx frame capture flag for one frame.
Core/Graphics/Source/DeviceContext.cpp	Forwards RequestCaptureNextFrame from context to impl.
Core/Graphics/InternalInclude/Babylon/Graphics/DeviceContext.h	Adds RequestCaptureNextFrame() to the DeviceContext contract.
Apps/Playground/Scripts/validation_native.js	Arms capture on the final render pass when test.capture is set.

[bghgary]

[Reviewed by Copilot on behalf of @bghgary]

LGTM