Make Color::withAlpha const

dgl/Color.hpp

@@ -68,7 +68,7 @@ struct Color {
    /**
       Create a new color based on this one but with a different alpha value.
     */
-    Color withAlpha(float alpha) noexcept;
+    Color withAlpha(float alpha) const noexcept;
 
    /**
       Create a color specified by hue, saturation and lightness.

dgl/src/Color.cpp

@@ -114,7 +114,7 @@ Color::Color(const Color& color1, const Color& color2, const float u) noexcept
     interpolate(color2, u);
 }
 
-Color Color::withAlpha(const float alpha2) noexcept
+Color Color::withAlpha(const float alpha2) const noexcept
 {
     Color color(*this);
     color.alpha = alpha2;