Add graphic.isDestroyed, prevent render crashes

[Geokureli]

Closes #2968

adds graphic.isDestroyed, detects when a destroyed graphic is being drawn, logs an error and displays the flixel logo, instead.

Note to @EliteMasterEric:

• I went with isDestroyed to match isLoaded and isDumped.
• I also use FlxG.log.error instead of throwing an error. I try to throw when I want the game to pause right when they did something wrong, not later, when I discover that something previously went wrong

[EliteMasterEric]

If you can successfully revert to rendering the Flixel logo instead of crashing, using Flx.error is way better than throwing, since gracefully handling errors is preferred whenever possible

[Geokureli]

using Flx.error is way better than throwing, since gracefully handling errors is preferred whenever possible

I mostly agree, there are some cases where I may want to be extremely loud about a dev mistake, especially if I can point to the exact offending line. But this issue is hard to predict, we can only detect late symptoms, so, yes

[Geokureli]

let me know if this works for you

here's my test case for posterity

package states;

import flixel.FlxG;
import flixel.FlxBasic;
import flixel.FlxSprite;

class DestroyedSpriteTestState extends flixel.FlxState
{
	var sprite1 = new FlxSprite(100, 100, "assets/images/haxe.png");
	var sprite2:FlxSprite;
	
	override function create()
	{
		super.create();
		
		add(new FlxSprite(0, 100));
		add(sprite2 = new FlxSprite(100, 0, "assets/images/haxe.png"));
		add(sprite1);
	}
	
	override function update(elapsed:Float)
	{
		if (FlxG.keys.justPressed.SPACE)
			sprite2.destroy();
		
		if (FlxG.keys.justPressed.ENTER)
			FlxG.switchState(new DestroyedSpriteTestState());
	}
	
}