Add convenience Color::withAlpha method

Signed-off-by: falkTX <falktx@falktx.com>

dgl/Color.hpp

@@ -65,6 +65,11 @@ struct Color {
     */
     Color(const Color& color1, const Color& color2, float u) noexcept;
 
+   /**
+      Create a new color based on this one but with a different alpha value.
+    */
+    Color withAlpha(float alpha) noexcept;
+
    /**
       Create a color specified by hue, saturation and lightness.
       Values must in [0..1] range.

dgl/src/Color.cpp

@@ -114,6 +114,13 @@ Color::Color(const Color& color1, const Color& color2, const float u) noexcept
     interpolate(color2, u);
 }
 
+Color Color::withAlpha(const float alpha) noexcept
+{
+    Color color(*this);
+    color.alpha = alpha;
+    return color;
+}
+
 Color Color::fromHSL(float hue, float saturation, float lightness, float alpha)
 {
     float m1, m2;