[babl] extensions/gggl{,-lies}.c: fix a Fixme: (>>8 vs /255)

[Massimo Valentini <mvalentini src gnome org>]

commit 0550e4016f904a5de43a194e6f5c735e7d894391
Author: Massimo Valentini <mvalentini src gnome org>
Date:   Sat Jul 14 17:07:38 2012 +0200

    extensions/gggl{,-lies}.c: fix a Fixme: (>>8 vs /255)
    
    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.

 extensions/gggl-lies.c |    7 ++++---
 extensions/gggl.c      |    7 ++++---
 2 files changed, 8 insertions(+), 6 deletions(-)
---
diff --git a/extensions/gggl-lies.c b/extensions/gggl-lies.c
index 849e3f4..d56a50e 100644
--- a/extensions/gggl-lies.c
+++ b/extensions/gggl-lies.c
@@ -1064,9 +1064,10 @@ conv_rgba8_rgbA8 (unsigned char *src, unsigned char *dst, long samples)
         }
       else
         {
-          dst[0] = (src[0] * src[3]) >> 8;      /* FIXME: check if this faster */
-          dst[1] = (src[1] * src[3]) >> 8;      /*        version (>>8 vs /255) */
-          dst[2] = (src[2] * src[3]) >> 8;      /*        is accurate enough   */
+#define div_255(a) ((((a)+127)+(((a)+127)>>8))>>8)
+          dst[0] = div_255 (src[0] * src[3]);
+          dst[1] = div_255 (src[1] * src[3]);
+          dst[2] = div_255 (src[2] * src[3]);
           dst[3] = src[3];
         }
       dst += 4;
diff --git a/extensions/gggl.c b/extensions/gggl.c
index 10094ac..f1fb8c0 100644
--- a/extensions/gggl.c
+++ b/extensions/gggl.c
@@ -1035,9 +1035,10 @@ conv_rgba8_rgbA8 (unsigned char *src, unsigned char *dst, long samples)
         }
       else
         {
-          dst[0] = (src[0] * src[3]) >> 8;      /* FIXME: check if this faster */
-          dst[1] = (src[1] * src[3]) >> 8;      /*        version (>>8 vs /255) */
-          dst[2] = (src[2] * src[3]) >> 8;      /*        is accurate enough   */
+#define div_255(a) ((((a)+127)+(((a)+127)>>8))>>8)
+          dst[0] = div_255 (src[0] * src[3]);
+          dst[1] = div_255 (src[1] * src[3]);
+          dst[2] = div_255 (src[2] * src[3]);
           dst[3] = src[3];
         }
       dst += 4;