NativeEngine: MultiRenderTarget framebuffers + OIT alpha blend modes

[bkaradzic-microsoft]

Paired engine PR: BabylonJS/Babylon.js#18568

What

Foundational native support for MultiRenderTarget (MRT) and the order-independent-transparency blend
modes, removing several "engine._gl is null" crash classes. It does not by itself turn the
MRT/OIT/FrameGraph validation tests green — those need further work (see Follow-ups).

Changes

• Plugins/NativeEngine/Source/NativeEngine.cpp / .h
  • CreateFrameBuffer and CreateMultiFrameBuffer now share a private
    CreateFrameBufferImpl(env, colorTextures, …) helper that builds one bgfx framebuffer with N color
    attachments (+ an optional depth/stencil attachment), so a MultiRenderTarget renders to all
    targets at once (bgfx writes every attachment of the bound framebuffer — no drawBuffers needed).
    The two N-API methods are kept for protocol compatibility and only differ in how they parse
    info[0] (single nullable texture vs. array); the single-RT path is just the 0-or-1-color case.
    The shared helper validates the attachment count against caps->limits.maxFBAttachments and keeps
    the "Stencil without depth…" warning and the isTextureValid assert.
  • Add alpha blend modes ALPHA_ONEONE_ONEONE (11) and ALPHA_LAYER_ACCUMULATE (17) used by the
    depth-peeling OIT renderer.

Paired engine PR

Pairs with the Babylon.js change (createMultipleRenderTarget + MRT helper overrides, applyStates
override, reverse-Z clear, alpha-mode mapping). That PR feature-detects both createMultiFrameBuffer
and the two alpha constants, so it degrades gracefully on native binaries that predate this PR.

CI

The validation suite uses the published babylonjs npm, which doesn't yet contain the paired JS, so
no new tests are enabled here and CI stays in the usual "pending dep bump" state. Verified locally
against a babylon.max.js built from the paired branch, and the plugin compiles clean
(cmake --build build/win32 --target NativeEngine --config RelWithDebInfo).

Follow-ups (separate work)

• OIT depth-peeling still faults inside the D3D11 driver on submit (multi-output / SRV↔RTV
  ping-pong); needs interactive GPU debugging (PIX / VS Graphics Debugger).
• Native does not yet apply the blend equation (MAX) — setAlphaEquation is a no-op — so OIT
  blending would still be incorrect after the crash is fixed.
• 2D-array / cube color attachments for MRT are approximated as 2D (test "MRT with different texture
  types").

---

Related PRs & landing order

• Babylon.js (engine / TS): [Native] MultiRenderTarget, reverse-Z clear, applyStates, OIT alpha modes Babylon.js#18568
• BabylonNative (C++ engine): NativeEngine: MultiRenderTarget framebuffers + OIT alpha blend modes #1754

These two are co-dependent and land in this order:

1. Babylon.js #18568 first — it only adds native-engine TS overrides, changes no WebGL behavior,
   re-enables no validation tests on its own, and feature-detects this PR's additions, so it can merge
   independently.
2. A babylonjs npm release ships that TS change.
3. BabylonNative NativeEngine: MultiRenderTarget framebuffers + OIT alpha blend modes #1754 last — after bumping the bundled babylonjs to that release. (This
   foundational pair does not yet turn the MRT/OIT/FrameGraph tests green — that is separate follow-up
   work.)

[bghgary]

[Reviewed by Copilot on behalf of @bghgary]

One refactor comment inline.

[bghgary]

[Reviewed by Copilot on behalf of @bghgary]

LGTM. The description is now stale, though: since #18568 feature-detects the native surface, the "co-dependent / landing order" framing no longer applies — the two can land separately, and this can come out of draft.

[Copilot]

Pull request overview

This PR adds foundational NativeEngine support for MultiRenderTarget framebuffers by creating a single bgfx framebuffer with N color attachments (plus optional depth/stencil), and exposes two additional alpha blend modes needed by OIT depth-peeling workflows. It also updates the bundled Babylon.js npm dependencies used by the Apps workspace.

Changes:

• Add createMultiFrameBuffer and refactor framebuffer creation into a shared CreateFrameBufferImpl that supports multiple color attachments.
• Expose additional alpha blend mode constants (ALPHA_ONEONE_ONEONE, ALPHA_LAYER_ACCUMULATE) for native-side blend state mapping.
• Bump Babylon.js-related npm dependencies in Apps/ to ^9.15.0 (and update lockfile).

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 1 comment.

File	Description
Plugins/NativeEngine/Source/NativeEngine.h	Declares new MRT framebuffer entrypoint and shared framebuffer helper.
Plugins/NativeEngine/Source/NativeEngine.cpp	Implements MRT framebuffer creation via a shared helper; adds OIT blend mode constants and JS binding for createMultiFrameBuffer.
Apps/package.json	Updates Babylon.js package dependency versions to ^9.15.0.
Apps/package-lock.json	Locks Babylon.js packages to 9.15.0 and updates related integrity metadata.

Files not reviewed (1)

• Apps/package-lock.json: Generated file

[bkaradzic-microsoft]

Fixed in 05862e2. The guard now reserves the depth/stencil slot only when one is generated: const uint32_t depthStencilCount = (generateStencilBuffer || generateDepth) ? 1u : 0u; then checks colorCount + depthStencilCount > caps->limits.maxFBAttachments. This matches the attachment-creation path below (which only appends the depth/stencil attachment inside the same generateStencilBuffer || generateDepth branch), so a max-color-count MRT with no depth/stencil is no longer wrongly rejected.