Playground validation: fix undefined renderImage in the scriptToRun path

[bkaradzic-microsoft]

Problem

loadPlayground declares its third parameter as referenceImage:

function loadPlayground(test, done, referenceImage, compareFunction) {

but the test.scriptToRun branch passes renderImage:

setTimeout(function () {
    try {
        currentScene = eval(scriptCode);
        processCurrentScene(test, renderImage, done, compareFunction);   // <-- not in scope

renderImage is not bound in that scope. It is only the parameter name of
processCurrentScene(test, renderImage, done, compareFunction). So the moment
a scriptToRun scene is evaluated, the call throws
ReferenceError: renderImage is not defined, the enclosing catch logs it,
and the test is marked failed.

Every other call site in loadPlayground already passes referenceImage,
and processCurrentScene forwards the argument straight through to
evaluateScreenshot(test, screenshot, renderImage, ...) where it is used as
the reference image to compare against. referenceImage is clearly the
intended value.

Why this was never noticed

The scriptToRun tests could not reach this line. They wait on
request.onreadystatechange, and the JsRuntimeHost XMLHttpRequest
polyfill never invokes it -- RaiseEvent only dispatches handlers registered
via addEventListener, and there are no on<event> property accessors at
all. So the request completed (readyState 4, status 200) but the
callback never ran and the test hung until the harness timeout.

That is fixed separately in BabylonJS/JsRuntimeHost#TBD. With the polyfill fixed,
these tests finally get as far as evaluating the scene -- and immediately hit
this ReferenceError.

Effect

Tests affected: Fog, Polygon, Lines, Lens, Self shadowing, GUI, Procedural
textures. They are currently excluded on every graphics API (D3D11, D3D12,
OpenGL, Vulkan, Metal, WebGPU) because they hang, so this is not WebGPU/Dawn
specific -- it affects every rendering backend.

With this fix plus the JsRuntimeHost one, locally:

Test	Before	After
Fog	hang	pass
Lines	hang	pass
Lens	hang	pass
Self shadowing	hang	pass
Polygon	hang	fails fast (earcut is not defined)
GUI	hang	fails fast (pixel diff 10.3%)
Procedural textures	hang	fails fast (name is not defined)

The three remaining failures are separate, unrelated issues that were previously
invisible; they now surface as fast, actionable errors instead of a hang.

[Copilot]

Pull request overview

Fixes a runtime ReferenceError in the native playground validation harness when running tests via the scriptToRun path by passing the correct in-scope reference image argument into processCurrentScene.

Changes:

• Replace an out-of-scope renderImage identifier with referenceImage in the test.scriptToRun execution branch.
• Align the scriptToRun branch with the other loadPlayground call sites that already pass referenceImage through to screenshot comparison.