Add HiDPI support for MacOS (Retina)

[shatyuka]

Description

Bring Retina support to MonoGame. Now we can render at full resolution instead of 1/2.

Details/Checklist

• Set SDL_WINDOW_ALLOW_HIGHDPI to SDL window
• Get the backing scale factor. Should be -[NSWindow backingScaleFactor]. But SDL_GL_GetDrawableSize() / SDL_GetWindowSize() is acceptable
• Convert between points and pixels (viewport/back buffer/mouse state)
• Only affects MacOS

What users should do

1. Set NSHighResolutionCapable to true in Info.plist.

<key>NSHighResolutionCapable</key>
<true/>

It's recommended to create a separate project for MacOS and set target framework to have the -macos suffix.

2. Set window size to a proper scale (like what we do in Windows).
   We can get scale factor from NSScreen.MainScreen.BackingScaleFactor.Value.

3. Handle screen change event and set back buffer size accoradingly.
   Insert the following code between using var game = new Game() and game.Run().

NSApplication.Init();
var window = NSApplication.SharedApplication.DangerousWindows[0];
NSNotificationCenter.DefaultCenter.AddObserver(NSApplication.DidChangeScreenParametersNotification, action, null);
NSNotificationCenter.DefaultCenter.AddObserver(NSWindow.DidChangeScreenNotification, action, window);

Related

#4802 #6936

[mrhelmut]

Would you mind rebasing this PR so that the build passes the checks? I'll proceed to reviewing it.