FlxFilterFrames.applyToSprite: add to, rather than wipe previous offset

[Geokureli]

Adjusts the current offset rather than ignoring its previous offset entirely. applyToSprite is typically used to add a glow or blur filter to an existing sprite sheet, typically it's desired for the hitbox to only cover the original graphical area so the offset is changed, but it assumed the original offset was 0,0. this change accounts for the previous value

fixes #2175

[Gama11]

Hm.. what if you apply multiple filters?

[Geokureli]

the number of filters nor the size of the filters supplied per FlxFilterFrame doesn't matter so much, the sprite's size will only increase by the amount specified, and the resulting graphic will always have the old graphic centered evenly within it.

This issue was discovered when my game calls FlxFilterFrame.fromFrames(...).applyToSprite(...) on the same sprite iteratively. the hitbox became more "off" with every iteration.

[Geokureli]

Thanks :)