More robust monitor selection and geometry

[metaquanta]

My laptop doesn't doesn't return a default screen. This will select the first (and probably only) screen in that case. It also has scale_factor=2. In that case this produces a window just filling the screen instead of one much, much larger.

[libredeb]

@metaquanta thank you for contribute. Your code look nice :+1: This topic had me crazy hahaha. Regards!