Splash screen is massive on Wayland compared to X11

### Tested versions

- Reproducible in: 4.3.dev cae759949

### System information

Godot v4.3.dev (cae759949) - Fedora Linux 39 (KDE Plasma) - Wayland - GLES3 (Compatibility) - NVIDIA GeForce RTX 4090 (nvidia; 545.29.06) - 13th Gen Intel(R) Core(TM) i9-13900K (32 Threads)

### Issue description

The splash screen is rescaled to match the window size, which makes it look massive on a 4K display:


This doesn't occur on X11 where the splash screen remains pixel-perfect. My Wayland display scale is 100%, and so is the editor scale.

Ideally, we should scale the splash screen according to the OS scale factor (or the editor scale factor) on both X11 and Wayland.

### Steps to reproduce

Use a Wayland session, enable the **Prefer Wayland** editor setting then restart the editor.

### Minimal reproduction project (MRP)

N/A

Start at the editor startup path reached after enabling the “Prefer Wayland” setting, and compare splash sizing under Wayland and X11. Trace where the splash screen is rescaled and where OS or editor scale factors are obtained; done means both sessions show an appropriately scaled splash screen without the reported 4K size discrepancy.

Written by the indexing model from the issue text.