shaders/colorspace: improve accuracy of desaturation

Instead of linearly desaturating, use a third-order polynomial
approximation.

src/shaders/colorspace.c

@@ -1964,8 +1964,12 @@ void pl_shader_color_map_ex(pl_shader sh, const struct pl_color_map_params *para
             GLSL("ipt.x = tone_map(ipt.x); \n");
         }
 
-        // Avoid raising saturation excessively when changing brightness
-        GLSL("ipt.yz *= min(i_orig / ipt.x, ipt.x / i_orig); \n");
+        // Avoid raising saturation excessively when raising brightness, and
+        // also desaturate when reducing brightness greatly to account for the
+        // reduction in gamut volume.
+        GLSL("vec2 hull = vec2(i_orig, ipt.x);                  \n"
+             "hull = ((hull - 6.0) * hull + 9.0) * hull;        \n"
+             "ipt.yz *= min(i_orig / ipt.x, hull.y / hull.x);   \n");
     }
 
     if (need_gamut_map) {