vo_opengl: use exactly the values defined by BT.709 for CMS

I could not see any difference whatsoever, but for usage with a 3DLUT
there's zero performance difference so we might as well follow the spec to
the letter.

Author: haasn

video/out/gl_lcms.c

@@ -157,14 +157,20 @@ struct lut3d *mp_load_icc(struct mp_icc_opts *opts, struct mp_log *log,
     if (!profile)
         goto error_exit;
 
-    cmsCIExyY d65;
-    cmsWhitePointFromTemp(&d65, 6504);
+    cmsCIExyY d65 = {0.3127, 0.3290, 1.0};
     static const cmsCIExyYTRIPLE bt709prim = {
         .Red   = {0.64, 0.33, 1.0},
         .Green = {0.30, 0.60, 1.0},
         .Blue  = {0.15, 0.06, 1.0},
     };
-    cmsToneCurve *tonecurve = cmsBuildGamma(NULL, 1.0/0.45);
+
+    /* Rec BT.709 defines the tone curve as:
+       V = 1.099 * L^0.45 - 0.099 for L >= 0.018
+       V = 4.500 * L              for L <  0.018
+
+       The 0.18 parameter comes from inserting 0.018 into the function */
+    cmsToneCurve *tonecurve = cmsBuildParametricToneCurve(NULL, 4,
+            (cmsFloat64Number[5]){1/0.45, 1/1.099, 0.099, 1/4.5, 0.18});
     cmsHPROFILE vid_profile = cmsCreateRGBProfile(&d65, &bt709prim,
                         (cmsToneCurve*[3]){tonecurve, tonecurve, tonecurve});
     cmsFreeToneCurve(tonecurve);