SubViewport only draws once, only in Safari Technology Preview

### Tested versions

- Reproducible in: 4.3 stable + Safari Tech Preview 199
- Not reproducible in: 4.3 stable + regular Safari or Chrome

### System information

Godot v4.3.stable - macOS 14.5.0 - GLES3 (Compatibility) - Apple M1 Max - Apple M1 Max (10 Threads)

### Issue description

It seems that a SubViewport will only draw successfully once—after that it keeps its contents indefinitely, apparently ignoring the `render_target_update_mode`.

### Steps to reproduce

1. Get the [2D in 3D demo project](https://godotengine.org/asset-library/asset/2803)
2. Export it for web
3. Open the resulting page in Safari Tech Preview

### Minimal reproduction project (MRP)

https://godotengine.org/asset-library/asset/2803

Start with the linked 2D in 3D demo project, export it for web, and reproduce the SubViewport behavior in Safari Technology Preview 199. Compare it with regular Safari and Chrome while checking render_target_update_mode; done means explaining why the viewport draws only once and confirming the behavior is corrected.

Written by the indexing model from the issue text.