Add BC3 texture for Xenoblade X

Author: Thealexbarney

XbTool/XbTool/Common/Textures/Decode.cs

@@ -37,6 +37,10 @@ public static byte[] DecodeTexture(this ITexture texture)
                     Swizzle.Deswizzle(texture, 3);
                     decoded = Dxt.DecompressDxt1(texture);
                     break;
+                case TextureFormat.BC3 when texture is Xbx.Textures.MtxtTexture tex:
+                    Xbx.Textures.Swizzle.Deswizzle(tex, 7);
+                    decoded = Dxt.DecompressDxt5(texture);
+                    break;
                 case TextureFormat.BC3:
                     Swizzle.Deswizzle(texture, 4);
                     decoded = Dxt.DecompressDxt5(texture);

XbTool/XbTool/Xbx/Textures/MtxtTexture.cs

@@ -44,6 +44,9 @@ public MtxtTexture(DataBuffer data)
                 case 49:
                     Format = TextureFormat.BC1;
                     break;
+                case 51:
+                    Format = TextureFormat.BC3;
+                    break;
                 default:
                     throw new NotImplementedException($"Texture format {Type}");
             }