Additional Subviewport Flicker Bug

[Flashfyre]

Tested versions

Reproduceable in LTS v26.1 and LTS v26.3 subviewport fix version (likely affects all versions)

System information

Windows 10 - Redot LTS v26.3 subviewport fix

Issue description

A strange flicker phenomenon with arbitrary textures in memory happens with subviewports under certain circumstances, see MRP.

This bug does NOT happen on compatibility mode.

It is important to know that this piece of code is key and without it, the flicker does not happen.

Background3D.gd
get_tree().create_timer(timeout).timeout.connect(queue_free)

Steps to reproduce

See MRP

Minimal reproduction project (MRP)

subviewport-bug-2.zip

The issue is partially fixed by #1316

[GeneralProtectionFault]

Attaching this modified MRP which has been illustrates the remaining issue.
EDIT: Will attach when Github stops failing to upload :-|

The worst is fixed, we don't have garbage data artifact with the PR merged, but when the child scenes are added (essentially, the image switches in the project), the first couple frames can appear blank.

This only happens on Vulkan because OpenGL 0-initializes textures. Essentially (Vulkan), I think the subviewport can sample the texture before it's actually rendered, when that texture is added in the same frame as said subviewport sampling.

The thing that makes it a clear issue is that it does not appear to be deterministic. Testing has shown the frames affected varies.