Leocad CLI render - Model space gradient forces background on render output

[nathaneltitane]

Describe the bug
Using Leocad to render a model, while the Background color is set to 'Gradient' results in a nontransparent PNG output for said render.

Furthermore, there is no cli argument to specify a background type/color selection, leaving the render routine vulnerable to whatever happens under the general LeoCAD config.

To Reproduce
Steps to reproduce the behavior:

1. Go to Preferences > Colors > Background
2. Click on Gradient
3. Run render routine of models via terminal emulator
4. See BG output to gradient color out rather than transparent.

Expected behavior
CLI commands should be complete and independant from UI app config to specify parameters according to needs and prevent undesired results.

Screenshots
If applicable, add screenshots to help explain your problem.

Version (please complete the following information):

• OS: debian 12 64 bit - KDE
• LeoCAD Version continuous X11