Add Premultiplied methods

This will allow me to write code to translate to/from premultiplied
colors when loading/saving formats


Former-commit-id: 278c8c79db008e019b595b4c3a410f641077184d
Former-commit-id: f9ea4706cb4c9f8b9857ec9d86de6c5a5b9306c6
Former-commit-id: 2e228c9e75548e448bc0701ee6532b8d5df724e5

src/ImageProcessor/Colors/Color.cs

@@ -8,7 +8,6 @@ namespace ImageProcessor
     using System;
     using System.ComponentModel;
     using System.Numerics;
-    using System.Runtime.CompilerServices;
 
     /// <summary>
     /// Represents a four-component color using red, green, blue, and alpha data.
@@ -508,6 +507,34 @@ namespace ImageProcessor
             return new Color(r, g, b, gamma.A);
         }
 
+        /// <summary>
+        /// Converts a non-premultipled alpha <see cref="Color"/> to a <see cref="Color"/>
+        /// that contains premultiplied alpha.
+        /// </summary>
+        /// <param name="r">The red component of this <see cref="Color"/>.</param>
+        /// <param name="g">The green component of this <see cref="Color"/>.</param>
+        /// <param name="b">The blue component of this <see cref="Color"/>.</param>
+        /// <param name="a">The alpha component of this <see cref="Color"/>.</param>
+        /// <returns>The <see cref="Color"/>.</returns>
+        public static Color FromNonPremultiplied(float r, float g, float b, float a)
+        {
+            return new Color(r * a, g * a, b * a, a);
+        }
+
+        /// <summary>
+        /// Converts a premultipled alpha <see cref="Color"/> to a <see cref="Color"/>
+        /// that contains non-premultiplied alpha.
+        /// </summary>
+        /// <param name="r">The red component of this <see cref="Color"/>.</param>
+        /// <param name="g">The green component of this <see cref="Color"/>.</param>
+        /// <param name="b">The blue component of this <see cref="Color"/>.</param>
+        /// <param name="a">The alpha component of this <see cref="Color"/>.</param>
+        /// <returns>The <see cref="Color"/>.</returns>
+        public static Color ToNonPremultiplied(float r, float g, float b, float a)
+        {
+            return new Color(r / a, g / a, b / a, a);
+        }
+
         /// <summary>
         /// Gets a <see cref="Vector4"/> representation for this <see cref="Color"/>.
         /// </summary>