fixed a bug of Screenshot on win7

src/Shared/HandyControl_Shared/Data/Enum/ImageCodecFlags.cs

@@ -0,0 +1,18 @@
+using System;
+
+namespace HandyControl.Data
+{
+    [Flags]
+    internal enum ImageCodecFlags
+    {
+        Encoder = 0x00000001,
+        Decoder = 0x00000002,
+        SupportBitmap = 0x00000004,
+        SupportVector = 0x00000008,
+        SeekableEncode = 0x00000010,
+        BlockingDecode = 0x00000020,
+        Builtin = 0x00010000,
+        System = 0x00020000,
+        User = 0x00040000
+    }
+}