Find the position of the first bit set in
mask, searching from (but not including)
Bits are numbered from 0 (least significant) to sizeof(ulong) * 8 - 1 (31 or 63, usually).
To start searching from the last bit, set
nth_bit to -1 or GLIB_SIZEOF_LONG * 8.
a ulong containing flags
the index of the bit to start the search from
the index of the first bit set which is lower than