Fix FluidSlotWidget fluidChanged detect

[Taskeren]

This is originally found because I'm working on an enhanced fluid tank in GT.
Because the fluid value is kept updated from SWidgetUpdate, I think there must be something wrong.

Then I found that in the method FluidSlotWidget#fluidChanged,
the cached value is also kept changing, whose fluid amount is jumping from x B to 2x B then back (eg 64B lava to 128B then 64B).

The true cause of the amount changing is the line in FluidTankLong#getFluidStack(), which the function is called in isFluidEqual:

if (internal.amount != lastFluidAmountInStack) {
    storedAmount -= lastFluidAmountInStack - internal.amount;
}

for example, storedAmount is 64B, but the lastFluidAmountInStack is 0, then it makes the amount added to 128B.

I'm not sure what does this lastFluidAmountInStack do, but removing it fixes this problem.