When generating single precision floats, Godot casts a uint32_t to float,
causing uniformity loss.
This new randf, inspired by T. R. Campbell's random_real, samples the output
of rand as the fraction part of an infinite binary number, with some tricks
to reduce ops and branching. This method provides "good enough" uniformity at
decent speed, for floats greater than 2^-64. Smaller numbers are floored to 0.
