coerce values to accumulator type for AddSource/SubSource

[Jamiras]

When combining values, they're automatically promoted to the most permissive data type (float is preferred over integer if either operand is a float), but the accumulator type is fixed by the first thing stored in it. So if the accumulator is an integer and a float value is subtracted, the accumulator was being changed into a float, the subtraction was performed, then the result was turned back into an integer to store in the accumulator.

Because subtracting a decimal value results in another decimal value slightly lower than if only the whole part was subtracted, forcing that result back to an integer results in a value one lower than if the integer conversion had occurred before the subtraction.

Given the expression:

    (int)(18 - 17.5)

If multiple coercions occur, the result is 0:

    (int)((float)18 - 17.5) => (int)(0.5) => 0

But if the coercion only appies to the modifier, the result is 1:

    18 - (int)17.5 => 18 - 17 => 1

Given the way the accumulator functions, I think the coercion should only be applied to the modifier.

This is also consistent with rcheevos prior to 12.0. More information is here: https://discord.com/channels/476211979464343552/1437468039162630244/1437589445355503717