Add the ability to change internal resolution of emulated games

I have a 720 display and steam-buddy configures on the fly retroarch cores to render at 1080.
My PC struggles in some games to render at a such high resolution ane changing the resolution to a lower one could improve the experience.
This modification could be made by adding an option in the settings menu or even just moving the configuration files to the home directory

No files, tests, or entry points are named. Start by locating how steam-buddy configures RetroArch cores and where the settings menu or configuration files are handled; done means users can select a lower internal rendering resolution and games use that setting successfully.

Written by the indexing model from the issue text.