Using the correct scene resolution in multi-monitor setups

Simply using the primary screen resolution obtained from Electron is sufficient when there's just one monitor. However, some cases fail when there are more displays.

Thus far, I haven't been able to track down a consistent way to determine the correct resolution of a given monitor (indexed by `obs-studio-node` via `0, 1, ...`) and use it for the scene.

Will make a post if I find something.