Make luminance Y public

src/image/luminance.rs

@@ -4,7 +4,7 @@ use ccvl_proc_macros::Pixel;
 
 #[derive(Clone, Copy, Debug, PartialEq, Pixel)]
 pub struct Luminance {
-	y: f32
+	pub y: f32
 }
 
 const R_COEFF: f32 = 0.2126;