QOA: Remove unnecessary memory allocation

[DeeJayLSP]

qoa_desc has a fixed size of 264 bytes, therefore no need to use a pointer.

*dec highly depends on the amount of channels and whether the result has more than 5120 samples per channel or not, so it remains variable.

This change shouldn't affect existing projects on vanilla Godot.

[DeeJayLSP]

Would it be useful to turn *dec into a Vector<int16_t> or a LocalVector<int16_t> too? This way the entire QOA implementation would have zero memallocs (at least from the class' side).

The entire original implementation was based on the reference implementation, so I didn't think of it.

Edit: done in another PR

[akien-mga]

Thanks!