bitNthMsf
Find the position of the first bit set in @mask, searching from (but not including) @nth_bit downwards. Bits are numbered from 0 (least significant) to sizeof(#gulong) * 8 - 1 (31 or 63, usually). To start searching from the last bit, set @nth_bit to -1 or GLIB_SIZEOF_LONG * 8.
Return
the index of the first bit set which is lower than @nth_bit, or -1 if no lower bits are set
Parameters
mask
a #gulong containing flags
nthBit
the index of the bit to start the search from