Use Haiku API's IsDark()

As it came with Haiku R1beta5.

src/Preferences.cpp

@@ -58,37 +58,21 @@ LoadPreferences()
 }
 
 
-bool
-IsDark(rgb_color color)
-{
-	// From http://alienryderflex.com/hsp.html
-	// Useful in particular to decide if the color is "light" or "dark"
-	// by checking if the perceptual brightness is > 127.
-	// TODO: Once Haiku Beta5 is out, we can use its IsDark()
-
-	int32 brightness
-		= (uint8)roundf(sqrtf(0.299f * color.red * color.red + 0.587f * color.green * color.green
-							  + 0.114 * color.blue * color.blue));
-
-	return brightness <= 127;
-}
-
-
 const float
 GetMutedTint(const rgb_color color, const CapitalBeMuted& type)
 {
 	switch (type) {
 		case CB_MUTED_TEXT:
 		{
-			return IsDark(color) ? B_DARKEN_2_TINT : B_LIGHTEN_1_TINT;
+			return color.IsDark() ? B_DARKEN_2_TINT : B_LIGHTEN_1_TINT;
 		}
 		case CB_MUTED_BG:
 		{
-			return IsDark(color) ? B_LIGHTEN_2_TINT : B_DARKEN_1_TINT;
+			return color.IsDark() ? B_LIGHTEN_2_TINT : B_DARKEN_1_TINT;
 		}
 		case CB_ALT_ROW:
 		{
-			return IsDark(color) ? 0.94 : 1.05;
+			return color.IsDark() ? 0.94 : 1.05;
 		}
 		default:
 			return B_NO_TINT;