Switch {map, apply}2_without_alpha Pixel functions to trait defaults

- This makes their implementations parallel with {map,
  apply}_without_alpha implementations
- Per PR feedback in image-rs#2239

src/color.rs

@@ -382,19 +382,6 @@ impl<T: $($bound+)*> Pixel for $ident<T> {
         }
     }
 
-    fn map2_without_alpha<F>(&self, other: &Self, f: F) -> $ident<T> where F: FnMut(T, T) -> T {
-        let mut this = (*self).clone();
-        this.apply2_without_alpha(other, f);
-        this
-    }
-
-    fn apply2_without_alpha<F>(&mut self, other: &$ident<T>, mut f: F) where F: FnMut(T, T) -> T {
-        const ALPHA: usize = $channels - $alphas;
-        for (a, &b) in self.0[..ALPHA].iter_mut().zip(other.0[..ALPHA].iter()) {
-            *a = f(*a, b)
-        }
-    }
-
     fn invert(&mut self) {
         Invert::invert(self)
     }

src/traits.rs

@@ -377,13 +377,21 @@ pub trait Pixel: Copy + Clone {
     /// of this pixel and ```other``` pairwise.
     fn map2_without_alpha<F>(&self, other: &Self, f: F) -> Self
     where
-        F: FnMut(Self::Subpixel, Self::Subpixel) -> Self::Subpixel;
+        F: FnMut(Self::Subpixel, Self::Subpixel) -> Self::Subpixel
+    {
+        let mut this = *self;
+        this.apply2_with_alpha(other, f, |x, _| x);
+        this
+    }
 
     /// Apply the function ```f``` to each channel except the alpha channel,
     /// of this pixel and ```other``` pairwise. Works in place.
     fn apply2_without_alpha<F>(&mut self, other: &Self, f: F)
     where
-        F: FnMut(Self::Subpixel, Self::Subpixel) -> Self::Subpixel;
+        F: FnMut(Self::Subpixel, Self::Subpixel) -> Self::Subpixel
+    {
+        self.apply2_with_alpha(other, f, |x, _| x);
+    }
 
     /// Invert this pixel
     fn invert(&mut self);