Debug pointer accounts for FlxSprite opacity

[EliteMasterEric]

When using the debugger cursor, if an FlxSprite had an alpha of 0, it would still select it. You'd also select a sprite if you clicked one of its transparent pixels. Both of these were annoying when trying to use the cursor to select a specific sprite, this fixes both of those issues.

[Geokureli]

When making PRs please have your settings.json include the following, to avoid reformatting entire files:

"[haxe]":
{
    "editor.formatOnSaveMode": "modifications"
}

and make sure your hxFormat.json has this, or simply disable autoformatting, altogether

"indentation":
{
    "trailingWhitespace": true
}

[Geokureli]

This doesn't work in my tests, closing for https://github.com/HaxeFlixel/flixel/pull/3184