TextureDecoder: Deduplicate some utility code

Source/Core/VideoCommon/TextureDecoder_Util.h

@@ -0,0 +1,25 @@
+// Copyright 2016 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include "Common/CommonTypes.h"
+
+struct DXTBlock
+{
+  u16 color1;
+  u16 color2;
+  u8 lines[4];
+};
+
+constexpr u32 MakeRGBA(int r, int g, int b, int a)
+{
+  return (a << 24) | (b << 16) | (g << 8) | r;
+}
+
+constexpr int DXTBlend(int v1, int v2)
+{
+  // 3/8 blend, which is close to 1/3
+  return ((v1 * 3 + v2 * 5) >> 3);
+}