Prepares a RangeOutput of length, carrying [0, keep) from prior — a Float64Array.set for the values and a byte-wise copy for the bitmap, with the straddling byte masked.
length
[0, keep)
prior
Float64Array.set
Prepares a RangeOutput of
length, carrying[0, keep)fromprior— aFloat64Array.setfor the values and a byte-wise copy for the bitmap, with the straddling byte masked.