Decrement clone counter only if it's clone

[apple502j]

Resolves

Resolves #505
Resolves #2352

Proposed Changes

Decrement clone counter only if it's clone

Reason for Changes

RenderedTarget.dispose is called when the target gets disposed somehow, including deleting a sprite or its clone. This makes the clone counter go below 0.

[adroitwhiz]

Any reason not to remove the decrement inside Sprite.removeClone (specifically, inside the if (cloneIndex >= 0) block, since removeClone is called regardless of whether sprites are clones)?