Wayland: Auto UI scale is rounded up when using fractional scaling

### Tested versions

v4.3.dev4.official.df78c0636

### System information

Fedora 39, Radeon 780M, GNOME Wayland

### Issue description

Godot does not follow the system display scale, which can result in it being way too big or way too small on some screens.

Display Scaling setting in Godot seems to only be able to automatically set itself to integer scales and it seems to be global, as in, it does not adjust the scale according to different displays in multi-monitor setups.

### Steps to reproduce

1. Run Godot with `--display-driver wayland` in a Wayland session
2. Depending on your display and scaling configuration, Godot's UI may or may not be too small or too big.

### Minimal reproduction project (MRP)

N/A

Start by reproducing the issue with `--display-driver wayland` in a GNOME Wayland session using fractional and multi-monitor scaling. Trace the display-scale handling from that Wayland entry point; done means Godot follows fractional system scales and adjusts correctly across different displays.

Written by the indexing model from the issue text.