bug: scale factor not correct on Windows

[jolexxa]

Godot does not have per-monitor DPI awareness (see godotengine/godot#56341), so we are not actually quite computing the scale factor correctly. I can prove this by connecting a primary 4K display at 150% and an HD screen at 100%. Godot reports the HD screen's dimensions incorrectly, scaled by 1.5 since the primary monitor is at 150%.

I am preparing a fix by determining the monitor's true native resolution (through some GDI32 functions, of all things -- the only way to get it reliably without per-monitor DPI awareness, it seems). This fix is in #7 but will need some more cleanup.