Fix FlxSpriteGroup origin (#2981)

* update children origins relative to their positions

* add originTransform unit test

flixel/group/FlxSpriteGroup.hx

@@ -1057,7 +1057,7 @@ class FlxTypedSpriteGroup<T:FlxSprite> extends FlxSprite
 		Sprite.offset.copyFrom(Offset);
 
 	inline function originTransform(Sprite:FlxSprite, Origin:FlxPoint)
-		Sprite.origin.copyFrom(Origin);
+		Sprite.origin.set(x + origin.x - Sprite.x, y + origin.y - Sprite.y);
 
 	inline function scaleTransform(Sprite:FlxSprite, Scale:FlxPoint)
 		Sprite.scale.copyFrom(Scale);

tests/unit/src/flixel/group/FlxSpriteGroupTest.hx

@@ -137,6 +137,33 @@ class FlxSpriteGroupTest extends FlxTest
 		Assert.isFalse(member2.revived);
 		return group;
 	}
+	@Test
+	/**
+	 * Ensure that member origins are correctly set when 
+	 * the group origin is set.
+	 */
+	function testOriginTransform()
+	{
+		var f1 = new FlxSprite(-10, 100);
+		var f2 = new FlxSprite(50, 50);
+		group.add(f1);
+		group.add(f2);
+
+		group.setPosition(280, 300);
+		group.origin.set(300, 400);
+
+		// Verify positions are updated - absolute
+		Assert.areEqual(270, f1.x);
+		Assert.areEqual(400, f1.y);
+		Assert.areEqual(330, f2.x);
+		Assert.areEqual(350, f2.y);
+
+		// Verify origins are correct - relative
+		Assert.areEqual(310, f1.origin.x);
+		Assert.areEqual(300, f1.origin.y);
+		Assert.areEqual(250, f2.origin.x);
+		Assert.areEqual(350, f2.origin.y);
+	}
 }
 
 class Member extends FlxSprite