Various FlxBar fixes

[Starmapo]

• Fixed frontFrames graphic not changing its useCount (for example, if you added an FlxBar and then called FlxG.bitmap.clearUnused(), the front graphic would be destroyed and the game would crash when trying to draw it)
• Render-method-specific variables are now destroyed correctly. It used to try to destroy variables that are only used in the opposite render method
• _frontFrame is now destroyed when being set to null
• Fixed _frontFrame documentation referring to it as an FlxSprite
• Removed redundant positionOffset = null code (the variable is already set to null earlier in the function)

[Geokureli]

Can you create an example state that illustrates the issues in the old code that this PR fixes

[Starmapo]

Can you create an example state that illustrates the issues in the old code that this PR fixes

Sure:

package;

import flixel.FlxG;
import flixel.FlxState;
import flixel.ui.FlxBar;

class PlayState extends FlxState
{
	var bar:FlxBar;

	override public function create()
	{
		bar = new FlxBar();
		bar.value = 50;
		bar.screenCenter();
		add(bar);

		super.create();
	}

	@:access(flixel.ui.FlxBar)
	override public function update(elapsed:Float)
	{
		if (bar != null)
		{
			if (FlxG.keys.justReleased.ONE)
			{
				// With the old code, the game will now crash when trying to draw the FlxBar,
				// as the front/filled graphic has been incorrectly cleared
				FlxG.bitmap.clearUnused();

				FlxG.log.add('Cleared unused graphics.');
			}
			else if (FlxG.keys.justReleased.TWO)
			{
				FlxG.log.add('Destroying the FlxBar...');

				final _frontFrame = bar._frontFrame;

				bar.destroy();

				// With the old code, these variables will not have been set to `null`
				if (FlxG.renderTile)
				{
					FlxG.log.add('frontFrames is null: ${bar.frontFrames == null}');
				}
				else
				{
					FlxG.log.add('_emptyBar is null: ${bar._emptyBar == null}');
				}

				// With the old code, _frontFrame will not have been destroyed
				FlxG.log.add('_frontFrame was destroyed: ${_frontFrame.sourceSize == null}');

				remove(bar, true);
				bar = null;
			}
		}

		super.update(elapsed);
	}
}

The last change (removed redundant positionOffset = null code) doesn't fix any bugs, just removes a redundant null set that already happens prior. (L199)

[Geokureli]

Can you shed some light on this change? was renderBlit a mistake, or was the comment above wrong? why frontFrames instead of _frontFrames?

[Starmapo]

_frontFrame and _filledFlxRect are only used in tile render mode, but the current code mistakenly tries to destroy these when the game is using blit render mode, and vice versa for the variables only used in blit mode. Simply changing line 201 to renderTile fixes it.

I changed line 203 to frontFrames as that value is left without being set to null in the current code, and set_frontFrames will also null out _frontFrame.

[Geokureli]

Thanks!