feat: default/unloaded-sprite

[amyspark-ng]

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It'd be nice if kaplay had a default/not-loaded sprite, so when an object is meant to have a sprite that is not loaded, it shows something else instead, so you know that the object is not being loaded but you can see the sprite, since sometimes one might not see the object and wonder but it's just the sprite that is not loaded

[lajbel]

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