Mouse size is 2x smaller than it should be on x11 w/ 2x scaling enabled (for HiDPI)

Nobody has claimed this yet.

## Description

The mouse cursor is 2x smaller than it should be and am unable to scale it up as a flatpak using env variables or overrides for xcursor scaling.

Things I tried were this..

```
sudo flatpak override --env=XCURSOR_THEME=Adwaita com.github.maoschanz.drawing
sudo flatpak override --env=XCURSOR_SIZE=72 com.github.maoschanz.drawing
flatpak run com.github.maoschanz.drawing

# attempt 2
env GDK_BACKEND="x11" GDK_SCALE=2 CLUTTER_SCALE=2 GDK_DPI_SCALE=2 flatpak run com.github.maoschanz.drawing

# a lot of things scaled up but not the mouse cursor, stayed the same size.
```

### Steps to reproduce the bug

1. Install Ubuntu 20.04 or newer (or Ubuntu Budgie) - login to x11 desktop
2. Runs these commands, logoff and back on. (best if you have a 4k laptop)

```
gsettings set org.gnome.settings-daemon.plugins.xsettings overrides "[{'Gdk/WindowScalingFactor', <2>}]"
gsettings set org.gnome.desktop.interface scaling-factor 2
```

3. Install drawing from a flatpak and observe small cursor.

## System

- Device (desktop computer/tablet/phone/...): MacBookPro11,4 1.0
- OS: Ubuntu 20.04.3 LTS x86_64
- Desktop environment: Budgie
- Package format: flatpak
- App version: 0.8.5

Reproduce the issue on Ubuntu 20.04 or newer in an X11 Budgie session with 2x scaling, using the listed gsettings commands and the Flatpak build. Compare cursor behavior with the reported XCURSOR_THEME, XCURSOR_SIZE, and scaling environment variables; done means the cursor renders at the expected HiDPI size in the Flatpak app.

Written by the indexing model from the issue text.