extensions/gggl{,-lies}.c: fix conversions u16 <-> u8

extensions/gggl{,-lies}.c: fix conversions u16 <-> u8

u16 maximum value is 0x10000 - 1 == (0x100 - 1) (0x100 + 1)
u8 maximum value is 0x100 - 1
thus the direct conversion requires multiplication/division per
0x100+1

division per 0x100+1 is implemented using 1 term of (1 + x)^-1
MacLaurin expansion

(1+x)^-1 == 1 + (-1) x

a * (1 + 1/256)^-1 * (256^-1) == (a - a / 256) / 256

with integer division rounded toward the nearest.