Add external texture array render test and fix numLayers bug

[bghgary]

Overview

This PR changes the ExternalTexture plugin from an asynchronous API to a synchronous API, adds thread safety, fixes a numLayers bug, and adds an end-to-end rendering test.

ExternalTexture API Change: Async to Sync

The main change replaces AddToContextAsync (which returned a Napi::Promise and required an extra render frame to resolve) with CreateForJavaScript (which synchronously returns a Napi::Value).

Before (async — required promises, callbacks, and an extra frame pump):

C++:

std::promise<void> textureCreationSubmitted{};
std::promise<void> textureCreationDone{};

auto externalTexture = std::make_shared<Babylon::Plugins::ExternalTexture>(d3d12Resource);

jsRuntime.Dispatch([&](Napi::Env env) {
    auto jsPromise = externalTexture->AddToContextAsync(env);
    env.Global().Get("setup").As<Napi::Function>().Call({
        jsPromise,
        Napi::Value::From(env, width),
        Napi::Value::From(env, height),
        Napi::Function::New(env, [&](const Napi::CallbackInfo&) {
            textureCreationDone.set_value();
        })
    });
    textureCreationSubmitted.set_value();
});

textureCreationSubmitted.get_future().get();

// Extra frame pump required for the texture to be created
m_update->Finish();
m_device->FinishRenderingCurrentFrame();
m_device->StartRenderingCurrentFrame();
m_update->Start();

textureCreationDone.get_future().get();

JS:

function setup(externalTexturePromise, width, height, textureCreatedCallback) {
    externalTexturePromise.then((externalTexture) => {
        const outputTexture = engine.wrapNativeTexture(externalTexture);
        scene.activeCamera.outputRenderTarget = new BABYLON.RenderTargetTexture(
            "rt", { width, height }, scene,
            { colorAttachment: outputTexture, generateDepthBuffer: true, generateStencilBuffer: true }
        );
        textureCreatedCallback();
    });
}

After (sync — no promises, no callbacks, no extra frame pump):

C++:

auto externalTexture = Babylon::Plugins::ExternalTexture{d3d12Resource};

jsRuntime.Dispatch([&](Napi::Env env) {
    auto jsTexture = externalTexture.CreateForJavaScript(env);
    env.Global().Get("setup").As<Napi::Function>().Call({
        jsTexture,
        Napi::Value::From(env, width),
        Napi::Value::From(env, height),
    });
});

JS:

function setup(externalTexture, width, height) {
    const outputTexture = engine.wrapNativeTexture(externalTexture);
    scene.activeCamera.outputRenderTarget = new BABYLON.RenderTargetTexture(
        "rt", { width, height }, scene,
        { colorAttachment: outputTexture, generateDepthBuffer: true, generateStencilBuffer: true }
    );
}

Other ExternalTexture Changes

• Fix numLayers bug: CreateForJavaScript passed hardcoded 1 for numLayers in Attach(). Changed to m_impl->NumLayers() so texture array metadata is preserved.
• Thread safety: Added std::scoped_lock to Width(), Height(), Get(), and Update().
• Simplified Update API: Removed layerIndex parameter — the full texture (all layers) is always updated.
• Texture lifecycle: Replaced handle-based tracking with texture-object tracking (AddTexture/RemoveTexture/UpdateTextures).

New: External Texture Array Render Test

• Tests.ExternalTexture.Render.cpp: Creates a 3-slice texture array (R/G/B), renders each slice via sampler2DArray shader to an external render target, verifies pixels.
• tests.externalTexture.render.ts: JS test with GLSL shaders sampling from texture array.
• RenderDoc.h/cpp: Optional GPU capture support (disabled by default).
• Platform Utils: CreateTestTextureArrayWithData, CreateRenderTargetTexture, ReadBackRenderTarget (D3D11, Metal; stubs for D3D12/OpenGL).

Testing

All 7 unit tests pass on Win32 (RelWithDebInfo).