Fix editor auto display scale on Windows

[timothyqiu]

The problem: I have my system display scale factor set to 175%. But the Godot editor fails to detect it, the UI is very small and the editor setting says Auto (100%).

The editor is currently using a strange way to determine the default scale:

godot/editor/settings/editor_settings.cpp

Lines 1832 to 1846 in d413181

	// Use the smallest dimension to use a correct display scale on portrait displays.
	const int smallest_dimension = MIN(DisplayServer::get_singleton()->screen_get_size(screen).x, DisplayServer::get_singleton()->screen_get_size(screen).y);
	if (DisplayServer::get_singleton()->screen_get_dpi(screen) >= 192 && smallest_dimension >= 1400) {
	// hiDPI display.
	return 2.0;
	} else if (smallest_dimension >= 1700) {
	// Likely a hiDPI display, but we aren't certain due to the returned DPI.
	// Use an intermediate scale to handle this situation.
	return 1.5;
	} else if (smallest_dimension <= 800) {
	// Small loDPI display. Use a smaller display scale so that editor elements fit more easily.
	// Icons won't look great, but this is better than having editor elements overflow from its window.
	return 0.75;
	}
	return 1.0;

My screen resolution is 2560x1600 and the DPI reported is 168. So the scale is indeed 100% according to this algorithm.

According to MSDN, the DPI reported by GetDpiForMonitor(type=MDT_DEFAULT) shows the correct factor for scaling UI elements (with 96 as 100%). I believe the default scale can be as easy as dividing screen_get_dpi() with 96.

In this PR, I also:

• optimized screen_get_dpi() to avoid enumerating the monitors twice
• changed the error return value of screen_get_dpi() from 72 to 96 as it means 100% scale on Windows
• removed an unnecessary optional parameter