[FEATURE] Add command-line option to specify a custom configuration file

Please confirm the following points:

• This feature request is NOT for the Android apps in the Play Store
• I have searched the project page to check if a similar request was already made

Application or Project

projectM Standalone SDL2 Frontend (includes the Steam app)

Is Your Enhancement Related to a Problem?

projectMSDL.properties is created in a specific directory, but there is no command line option to override it.

https://github.com/projectM-visualizer/frontend-sdl-cpp/blob/master/README.md?plain=1#L94-L101

Your Suggested Enhancement

Add a command line option to specify a projectMSDL.properties file. Since there is a save option inside the projectMSDL app, may need to check if a config file was passed in, and make a decision whether changes should be saved to the currently loaded file. Perhaps the user could be provided the context of what save file is currently loaded.

I think also maybe copying the default file with the helpful commented out setting descriptions to the user file might be helpful.

Alternative Solutions

No response

Additional Context

No response

Start with the configuration-file behavior described in the README at lines 94-101, then locate how projectMSDL.properties is found, loaded, and saved. Determine the command-line option and define how an explicitly supplied file interacts with in-app saving. Done means the standalone SDL2 frontend accepts a custom configuration path and handles saving to the intended file.