STALKER 2 fix: Slate: find the viewport when UE5.5 passes all windows as one array

[Remleo]

This is a quick fix I made to get STALKER 2 (UE 5.5.4) working. You probably know a better way to solve this; I hope this PR at least helps point at the problem.

Depends on praydog/UESDK#3, which adds sdk::slate::locate_windows_array_offsets. This PR does
not bump the submodule, since the commit is not in your repository yet. It builds once that one is merged
and the submodule is updated.

On Stalker 2 (UE 5.5.4) the hooked draw function takes a TConstArrayView of per-window args in a3, not
FViewportInfo&. a4 does not match the other 5.5 variant either (a4[0] == renderer). So neither path
found the viewport, the UI was never moved to its own layer, and nothing showed in the headset:

[SlateRHIRenderer::DrawWindow_RenderThread] a4 is not UE 5.5 variant!
Failed to find FViewportInfo::GetRenderTargetProvider offset!
No viewport RT provider, skipping!

The change. When a4 is not the 5.5 variant, read a3 as {data, num} and take the viewport of the
first window, using the offsets UESDK reads from the code. After that it is the existing
slate_viewport->GetViewportRenderTargetTexture() path, nothing new.

Each step checks for a live object and falls back to the old path if anything is off:

• the array data is in the heap, not inside a module
• the window has a vtable inside a module
• the viewport has a vtable inside a module

The heap check keeps builds that pass FViewportInfo& in a3 on their old path. There the first field is a
vtable inside the image, so the check rejects it before anything is dereferenced.

Beware of num. There is no capacity after it, since this is an array view and not a TArray. My first
version bounded num by that word, measured it at 0 on some frames and dropped those frames. It is now
bounded by a constant (64 windows).

Tested

Release build, Quest 3 over Virtual Desktop, OpenXR, D3D12.

game	UE	a3	result
Stalker 2	5.5.4	windows array	UI shows, no rejected frames over a session
The Outer Worlds 2	5.4	FViewportInfo&	unchanged, FViewportInfo offset 0xe8 as before