Add modulo/remainder function to fx

[rvwagner]

Description

Add the % operator to fx. Its absence can often be worked around with if statements (or, in one memorable case, abs(sin(sample*rads(90))) in place of a hypothetical sample%2), but having the real operator would be nice. My use cases have generally been selecting out specific sets of lines and/or samples to do other calculations on (e.g. modifying only the even columns, or operating only on the blue pixels of a Bayer-pattern grid), so I wouldn't object to it being limited to integers, but there are certainly extended versions that work for floating-point data, which might be easier than dealing with type-checking.