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
  • NativeEngine::CreateMultiFrameBuffer: build one bgfx framebuffer with N color attachments
    (+ optional depth) so a MultiRenderTarget renders to all targets at once (bgfx writes every
    attachment of the bound framebuffer — no drawBuffers needed). The JS-controlled attachment count
    is validated against caps->limits.maxFBAttachments.
  • 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).

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.

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, and re-enables no validation tests on its own, 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.)