Respect Window.AllowAltF4 For SDLGamePlatform

[AristurtleDev]

Description

Currently Window.AllowAltF4 is not respected for SDLGamePlatform. No matter the property setting ALT+F4 will close the window.

This PR adds the check for Alt+F4 during the exit check in the platform loop and handles exiting based on the keyboard key combination and the Window.AllowAltF4 property. If the keyboard combination was not pressed, then it falls back to the default behavior of allowing the exit to occur.

Notes

Press Alt+F4 will trigger SDL.EventType.Quit

MonoGame/MonoGame.Framework/Platform/SDL/SDLGamePlatform.cs

Lines 109 to 111 in 40452a4

	case Sdl.EventType.Quit:
	_isExiting++;
	break;

However, Alt+F4 can also trigger Sdl.Window.EventId.Close

MonoGame/MonoGame.Framework/Platform/SDL/SDLGamePlatform.cs

Lines 227 to 229 in 40452a4

	case Sdl.Window.EventId.Close:
	_isExiting++;
	break;

It is inconsistent on which of these events is triggered first. Regardless, when both events are triggered, they _keys collection does not contain both the Keys.LeftAlt/Keys.RightAlt and the Keys.F4 due to the events firing before the keyboard update is performed.

Due to these reasons, this is why the check was moved into the platform loop and made during the _isExiting check.