Add alpha premultiplication.

Quik/Media/Color/FormatConvert.cs

@@ -4,6 +4,56 @@ namespace Quik.Media.Color
 {
     public static class FormatConvert
     {
+        public static void Premultiply(QImageLock image)
+        {
+            switch (image.Format)
+            {
+            case QImageFormat.RaF:
+            case QImageFormat.RaU8:
+            case QImageFormat.RgbF:
+            case QImageFormat.RgbU8:
+            case QImageFormat.RgbaF:
+            case QImageFormat.RgbaU8:
+                break;
+            default:
+                return;
+            }
+
+            int count = image.Width * image.Height * image.Depth;
+
+            if (image.Format.IsFloat())
+            {
+                LockIOF io = new LockIOF(image);
+
+                for (int i = 0; i < count; i++)
+                {
+                    QColorF color = io[i];
+
+                    color.R *= color.A;
+                    color.G *= color.A;
+                    color.G *= color.A;
+
+                    io[i] = color;
+                }
+            }
+            else
+            {
+                LockIO io = new LockIO(image);
+
+                for (int i = 0; i < count; i++)
+                {
+                    QColor color = io[i];
+                    float a = color.A/255.0f;
+
+                    color.R = (byte)(color.R * a);
+                    color.G = (byte)(color.G * a);
+                    color.B = (byte)(color.B * a);
+
+                    io[i] = color;
+                }
+            }
+        }
+
         public static void Convert(QImageLock dst, QImageLock src)
         {
             if (dst.Format.IsU8() && src.Format.IsU8())