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[sfnt] Improve sRGB constants.

* src/base/ftbitmap.c (ft_gray_for_premultiplied_srgb_bgra): Use slightly
more precise values.

Fixes #1018.

git/fs: mount .git/fs: mount/attach disallowed

--- a/src/base/ftbitmap.c

+++ b/src/base/ftbitmap.c

@@ -480,7 +480,7 @@

      * A gamma of 2.2 is fair to assume.  And then, we need to

      * undo the premultiplication too.

      *

-     *   https://accessibility.kde.org/hsl-adjusted.php

+     *   http://www.brucelindbloom.com/index.html?WorkingSpaceInfo.html#SideNotes

      *

      * We do the computation with integers only, applying a gamma of 2.0.

      * We guarantee 32-bit arithmetic to avoid overflow but the resulting

@@ -488,9 +488,9 @@

      *

      */

-    l = (  4732UL /* 0.0722 * 65536 */ * bgra[0] * bgra[0] +

-          46871UL /* 0.7152 * 65536 */ * bgra[1] * bgra[1] +

-          13933UL /* 0.2126 * 65536 */ * bgra[2] * bgra[2] ) >> 16;

+    l = (  4731UL /* 0.072186 * 65536 */ * bgra[0] * bgra[0] +

+          46868UL /* 0.715158 * 65536 */ * bgra[1] * bgra[1] +

+          13937UL /* 0.212656 * 65536 */ * bgra[2] * bgra[2] ) >> 16;

     /*

      * Final transparency can be determined as follows.