Fix integer overflow in intermediate values in QuantizerWu.kt

[RuurdBijlsma]

There's integer overflow issues in the Kotlin implementation of QuantizerWu.

Example in the maximize function (original, wrong, code):

tempNumerator = (halfR * halfR + halfG * halfG + halfB * halfB).toDouble()

I put in an image to be quantized, and these are the values:

halfR = 6645743
halfG = 5064075
halfB = 3940401

These variables are all int 32, so the above formula would cause integer overflow (halfR * halfR is already too much for i32).

Similar cases in other places. I fixed it in this PR by converting to double earlier in the flow.

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