[gegl] saturation-yuv: use exact values for CIE u/v whitepoint

[Øyvind "pippin" Kolås <ok src gnome org>]

commit f024ca1beac49e1f29c996695f5f832be871ff1f
Author: Øyvind Kolås <pippin gimp org>
Date:   Sun Jun 30 23:18:29 2019 +0200

    saturation-yuv: use exact values for CIE u/v whitepoint

 operations/workshop/saturation-yuv.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/operations/workshop/saturation-yuv.c b/operations/workshop/saturation-yuv.c
index 7611aa518..2810392d3 100644
--- a/operations/workshop/saturation-yuv.c
+++ b/operations/workshop/saturation-yuv.c
@@ -60,8 +60,8 @@ process (GeglOperation       *operation,
   glong i;
   float scale = o->scale;
 
-#define CIE_u_origin     0.209167f
-#define CIE_v_origin     0.488098f
+#define CIE_u_origin     (4/19.0f)
+#define CIE_v_origin     (9/19.0f)
 
   for (i = 0; i < n_pixels; i++)
     {