"Draw user backgrounds" and background image selection only works for the default wallpapers.

 * slick-greeter version 2.0.3
 * Distribution - Mint 21.3
 * Graphics hardware *and* driver used:
 `Device-1: Intel Skylake GT2 [HD Graphics 520] vendor: Hewlett-Packard driver: i915 v: kernel
    ports: active: eDP-1 empty: DP-1,HDMI-A-1,HDMI-A-2 bus-ID: 00:02.0 chip-ID: 8086:1916
  Device-2: AMD Topaz XT [Radeon R7 M260/M265 / M340/M360 M440/M445 530/535 620/625 Mobile]
    vendor: Hewlett-Packard driver: amdgpu v: kernel pcie: speed: 2.5 GT/s lanes: 4 bus-ID: 01:00.0
    chip-ID: 1002:6900
  Device-3: Lite-On HP HD Camera type: USB driver: uvcvideo bus-ID: 1-6:3 chip-ID: 04ca:7053
  Display: x11 server: X.Org v: 1.21.1.4 driver: X: loaded: amdgpu,ati,modesetting
    unloaded: fbdev,vesa gpu: i915 display-ID: :0 screens: 1
  Screen-1: 0 s-res: 1366x768 s-dpi: 96
  Monitor-1: eDP-1 model: Chi Mei Innolux res: 1366x768 dpi: 101 diag: 394mm (15.5")
  OpenGL: renderer: Mesa Intel HD Graphics 520 (SKL GT2) v: 4.6 Mesa 23.3.3 direct render: Yes`
 * 64 bit

Issue

1. I have "draw user backgrounds" selected in the Login Window settings. It's supposed to automatically display the wallpaper from your desktop on the login screen. It only does this with default wallpapers.

2. If I select background for the login screen manually, it will just be black screen, unless it's one of the default images. I suspect the issue here is that the Slick Greeter is somehow unable to access files in the home/ folders, and can only access those in the root folders.

I've had this issue on Mint 21.1 and Mint 21.2. Right now I'm on 21.3 and it persists. At first I thought it doesn't draw any wallpapers (because I always select one of my own right away), but this time I noticed that the default wallpapers do work. And so does the Login background (the black and white Mint logo wallpaper).

Steps to reproduce

1. Select a custom wallpaper from your home/ folders. Enable "draw user backgrounds" in the Login Window settings.
2. Manually select custom background from your home/ folders for the Login screen from your Login Window settings.

Expected behaviour

1. The Login screen will have your desktop wallpaper as a background
2. The Login screen will have whichever image you selected as a background

No files or tests are named. Reproduce on slick-greeter 2.0.3 with a custom wallpaper in the home directory, testing both user-background mode and manual background selection. Trace the greeter's background loading and home-directory access; done means custom images display in both cases, not only default wallpapers.