Turn FIXED_TO_FLOAT into a function

Monster Iestynrequested to merge
fixed2float-funcs into master

Turn FIXED_TO_FLOAT and FLOAT_TO_FIXED into the functions FixedToFloat and FloatToFixed, though keep the old macros for backwards compatibility. This fixes issues in !861 (merged) and makes !867 (closed) obsolete.

This really has no gameplay implications at all, this is just coding cleanup.

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