extensions/gggl{,-lies}.c: round toward nearest in integer unpremultiplication

extensions/gggl{,-lies}.c: round toward nearest in integer unpremultiplication

from (docs/BablFishPath.html):
../extensions/.libs/gggl-lies.so 0: RaGaBaA u8 to RGBA u8
    error: 0.001425 cost: 81 processings: 1 pixels: 128
../extensions/.libs/gggl.so 0: R'aG'aB'aA u8 to R'G'B'A u8
    error: 0.001971 cost: 71 processings: 1 pixels: 128
../extensions/.libs/gggl.so 0: R'aG'aB'aA u8 to R'G'B' u8
    error: 0.001971 cost: 71 processings: 1 pixels: 128

to:
../extensions/.libs/gggl-lies.so 0: RaGaBaA u8 to RGBA u8
    error: 0.000031 cost: 81 processings: 1 pixels: 128
../extensions/.libs/gggl.so 0: R'aG'aB'aA u8 to R'G'B'A u8
    error: 0.000042 cost: 61 processings: 1 pixels: 128
../extensions/.libs/gggl.so 0: R'aG'aB'aA u8 to R'G'B' u8
    error: 0.000042 cost: 61 processings: 1 pixels: 128